A sequence of layers of the Earth atmosphere is given as Troposphere, Stratosphere, Mesosphere, ?. Choose the correct term from the options that completes this sequence of major atmospheric layers in the correct increasing height order.

Introduction / Context:
This question tests basic general science knowledge about the vertical structure of the Earth atmosphere. The atmosphere is divided into several distinct layers based on temperature variation with altitude. Knowing the correct order of these layers is important in geography and environmental science topics.

Given Data / Assumptions:

• The given partial sequence is Troposphere, Stratosphere, Mesosphere, ?
• All options refer to well known atmospheric regions or related terms.
• We assume the standard textbook ordering of atmospheric layers from the Earth surface upward.

Concept / Approach:
The primary layers of the atmosphere, from the Earth surface upward, are:

• Troposphere: lowest layer, where weather phenomena occur.
• Stratosphere: contains the ozone rich region and jet streams.
• Mesosphere: middle region where many meteors burn up.
• Thermosphere: very high layer with significant temperature increase.
• Exosphere: outermost diffuse layer merging into space.

Therefore, after Mesosphere the next major named layer is Thermosphere.

Step-by-Step Solution:
Step 1: Start from the ground. The Troposphere is the first layer directly above the surface. Step 2: Above the Troposphere lies the Stratosphere. The boundary region between them is called the Tropopause, but the Stratosphere is the next full layer. Step 3: Above the Stratosphere is the Mesosphere, characterised by decreasing temperature with height again. Step 4: Above the Mesosphere comes the Thermosphere, where temperature increases sharply due to absorption of high energy solar radiation. Step 5: The Exosphere lies above the Thermosphere and gradually merges into outer space. Therefore in the sequence Troposphere, Stratosphere, Mesosphere, the immediate next layer is Thermosphere.

Verification / Alternative check:
A common mnemonic is to remember the order as T S M T E, standing for Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere. Applying this mnemonic directly gives Thermosphere as the missing term after Mesosphere.

Why Other Options Are Wrong:
Exosphere is a real atmospheric layer but it comes after the Thermosphere, not immediately after the Mesosphere. Tropopause is a boundary zone between Troposphere and Stratosphere, not a full major layer above the Mesosphere. Ozone Layer refers mainly to the ozone rich region inside the Stratosphere, not a separate layer placed above the Mesosphere. Ionosphere is a functional region overlapping parts of the Mesosphere and Thermosphere based on ionisation levels, not a distinct named layer in the basic school level sequence.

Common Pitfalls:
Students often confuse boundaries like the Tropopause with full layers, or mix up the order of Mesosphere and Thermosphere. It is helpful to memorise the main sequence as Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere and understand that some named features such as the Ozone Layer sit inside these layers rather than replacing them.

Final Answer:
The next major atmospheric layer after the Mesosphere is the Thermosphere.