The atmosphere around the earth always has which of the following as an essential component?

Introduction / Context:
This question asks you to identify what is fundamentally present in the atmosphere. The term atmosphere in basic science refers to the envelope of gases surrounding the earth. While this gas mixture may contain various components such as oxygen, nitrogen, moisture and dust, the question asks which of the given options is always associated with atmosphere, regardless of place or weather conditions.

Given Data / Assumptions:

• The subject is the atmosphere, meaning the gaseous envelope around the earth.
• The options are oxygen, air, moisture and dust.
• We assume standard school-level definitions in environmental science.
• We must identify a component that the atmosphere must always have to be called an atmosphere.
• Local variations such as pollution, humidity and composition may exist.

Concept / Approach:
The atmosphere is defined as a collection of gases, predominantly nitrogen and oxygen, along with small amounts of other gases. This mixture of gases in a given region is usually called air. Oxygen is an important part of air but not the only component, and in some planetary atmospheres oxygen may be negligible. Moisture and dust are variable components; humidity can be extremely low and dust content can vary with location and conditions. Therefore, the term air, meaning the gaseous mixture itself, best represents what the atmosphere must always contain.

Step-by-Step Solution:
Step 1: Consider oxygen. It is a major component of earth’s atmosphere, but the proportion of oxygen can vary, and in some planetary atmospheres oxygen is nearly absent. The concept of atmosphere does not depend on oxygen alone. Step 2: Consider air. Air is the name given to the mixture of gases present in the earth’s atmosphere. Without air or an equivalent gas mixture, there would not be an atmosphere as normally understood. Step 3: Consider moisture. Moisture in the form of water vapour varies widely with weather and climate, and extremely dry air is still part of the atmosphere. Step 4: Consider dust. Dust and particulates can be present in differing amounts, but a perfectly clean atmosphere without dust is still an atmosphere. Step 5: Recognise that air, the gaseous mixture, is the essential characteristic of atmosphere according to school-level definitions.

Verification / Alternative check:
Imagine a very dry, clean atmosphere over a polar region. Moisture can be extremely low, and dust content may be minimal, yet the region clearly has an atmosphere. Oxygen could technically be replaced by other gases in a different planet’s atmosphere, and we would still call that envelope an atmosphere. What must remain is a layer of gases—air in the broad sense. This confirms that air is the core concept that is always associated with atmosphere in basic science, even though its precise composition can change.

Why Other Options Are Wrong:
Oxygen is important on earth, but an atmosphere could theoretically exist without it, and the definition of atmosphere does not demand oxygen specifically. Moisture is not always present in significant amounts and can be nearly absent in very dry conditions. Dust is not necessary for the existence of an atmosphere and can vary greatly with pollution and environmental conditions.

Common Pitfalls:
Students often select oxygen because they strongly associate it with breathing and life. Others may choose moisture thinking of clouds and weather. The key is to focus on what defines the atmosphere itself, not on what makes it comfortable for humans. In reasoning questions, always distinguish between necessary conditions and common but nonessential features. Here, the necessary feature is the presence of air, the gaseous mixture, rather than any specific gas or additional component.

Final Answer:
The atmosphere always has air.