In the structure of the Earth's atmosphere, which layer is characterised by the presence of large numbers of free electrons and positive ions produced by solar radiation, and therefore plays a major role in radio communication?

Introduction / Context:
The Earth's atmosphere is divided into several layers, each with different properties, temperatures and functions. Some layers are important for weather, some for ozone protection and others for radio communication. This question focuses on the atmospheric layer that contains many electrons and positive ions, created mainly by the action of solar ultraviolet and X ray radiation on gas molecules. Knowing the names and functions of these layers is a common topic in general science and geography.

Given Data / Assumptions:

• The layer being described has a high concentration of free electrons and positive ions.
• These ions and electrons are produced by the ionisation of atmospheric gases by solar radiation.
• This layer strongly affects the propagation of radio waves.
• The options list common atmospheric layers: troposphere, ionosphere, stratosphere and mesosphere.
• We assume standard textbook definitions of these layers.

Concept / Approach:
The ionosphere is the region of the upper atmosphere where solar radiation ionises atoms and molecules, creating a partly ionised gas containing free electrons and positive ions. It extends roughly from about 60 kilometres to several hundred kilometres above the Earth's surface and overlaps parts of the mesosphere and thermosphere. The presence of charged particles in this region allows it to reflect or refract certain radio waves, particularly shortwave and medium frequency signals, back towards the Earth, enabling long distance radio communication. The troposphere is the lowest layer where weather occurs, the stratosphere contains the ozone layer and the mesosphere lies above the stratosphere but is not specifically defined by strong ionisation in basic textbooks.

Step-by-Step Solution:
Step 1: Identify the key features mentioned in the question: presence of free electrons and positive ions and partial composition of the layer by these charged particles. Step 2: Recall that the region of the atmosphere where solar ultraviolet and X ray radiation ionises gases is called the ionosphere. Step 3: Note that the troposphere is mainly associated with weather phenomena and contains most of the atmospheric water vapour but is not defined by strong ionisation. Step 4: Recognise that the stratosphere is known for the ozone layer and relatively stable air, and the mesosphere is higher still and noted for meteor burning, not primarily for its ions. Step 5: Conclude that the only option that accurately matches a layer composed partly of electrons and positive ions is the ionosphere.

Verification / Alternative check:
Standard geography and physics references describe the ionosphere as a region of the upper atmosphere that is significantly ionised and is divided into sub layers such as the D, E and F layers. These sources also explain that because of the abundance of free electrons, the ionosphere reflects certain radio frequencies, making long distance radio broadcasting possible. Troposphere is described as extending up to about 8 to 15 kilometres, stratosphere up to about 50 kilometres with ozone, and mesosphere above that, none of which are primarily defined by ionisation in school level discussions. This confirms that the question is referring to the ionosphere.

Why Other Options Are Wrong:
Option a, troposphere, is the lowest layer where clouds form and weather occurs, but it does not contain a high density of ions and free electrons as its defining feature.

Option c, stratosphere, is known mainly for containing the ozone layer, which absorbs ultraviolet radiation, but it is not described as being filled with charged particles for radio communication.

Option d, mesosphere, lies above the stratosphere and is where many meteors burn up, but it is not the main ionised region highlighted in basic radio communication topics.

Common Pitfalls:
Learners sometimes confuse the ionosphere with the stratosphere because both are linked to phenomena involving the Sun. The stratosphere has the ozone layer, which blocks ultraviolet light, while the ionosphere deals with ionisation and radio waves. Another pitfall is to think that any high layer in the atmosphere could be the ionosphere without remembering the specific function and name. To avoid confusion, remember that ionosphere contains ions, and the word itself hints at ionisation and its role in reflecting radio waves.

Final Answer:
The layer of the atmosphere that is partly composed of electrons and positive ions is the Ionosphere.