Depletion of the ozone layer in the upper atmosphere is mainly caused by which of the following substances?

Introduction / Context:
This environmental science question asks about ozone layer depletion in the stratosphere and the substances responsible for it. The ozone layer protects life on Earth by absorbing most of the harmful ultraviolet radiation from the sun. Damage to this layer has been a major global environmental issue, leading to international agreements such as the Montreal Protocol. Understanding which chemicals cause ozone depletion is essential for any basic study of environmental protection.

Given Data / Assumptions:

The focus is on depletion of the ozone layer in the upper atmosphere, not on ground level ozone pollution.
The substances listed include CFCs, carbon tetrachloride CCl4 and HCFCs.
All these chemicals can release chlorine atoms or related reactive species into the stratosphere.
Only one option correctly summarises the combined effect of all these substances on ozone.

Concept / Approach:
Chlorofluorocarbons, carbon tetrachloride and hydrochlorofluorocarbons are all man made chemicals once widely used in refrigeration, aerosols and industrial processes. When they reach the stratosphere, ultraviolet radiation breaks them down, releasing chlorine atoms. These chlorine atoms participate in catalytic cycles that destroy ozone molecules. Because more than one type of chlorine containing chemical contributes to the problem, the correct answer must include all of them rather than picking only one.

Step-by-Step Solution:
Step 1: Recall that chlorine containing industrial chemicals are the main cause of stratospheric ozone depletion.Step 2: Recognise that CFCs were historically the most famous example and were heavily regulated under international agreements.Step 3: Note that carbon tetrachloride and HCFCs also contain chlorine and can contribute to ozone destruction when they break down in the upper atmosphere.Step 4: Evaluate option D, All of the above, and see that it correctly combines the effect of all chemical groups listed.Step 5: Evaluate options A, B and C individually, which would wrongly suggest that only one particular compound group is responsible.Step 6: Conclude that All of the above is the best answer for an exam level description.

Verification / Alternative check:
You can verify this by recalling that international environmental agreements such as the Montreal Protocol target several families of ozone depleting substances, not just one. Regulatory lists typically include CFCs, carbon tetrachloride, halons, HCFCs and other related chemicals. The reason is that all these compounds release reactive halogen atoms under ultraviolet radiation. Although some newer compounds are designed to be less harmful, the overall principle remains that multiple chlorine containing chemicals contribute to ozone loss. Therefore, any answer that mentions only one group is incomplete.

Why Other Options Are Wrong:
Option A, CFCs chlorofluorocarbons, is incomplete. CFCs are a major cause but not the only chemical type that depletes ozone.
Option B, Carbon tetrachloride CCl4, is also incomplete. It is an ozone depleting substance, but the question lists more than one harmful compound.
Option C, HCFCs, is wrong as a stand alone answer because it ignores the important role of CFCs and carbon tetrachloride that are also clearly mentioned in the options.

Common Pitfalls:
Many students remember only CFCs due to famous campaigns against aerosol sprays and old refrigerators and therefore choose only that option. Another pitfall is to underestimate the role of lesser known industrial solvents such as carbon tetrachloride. In multiple choice questions, when several options list different harmful variants and one option groups them logically, it is often correct to choose the combined option if it matches scientific facts.

Final Answer:
The correct answer is All of the above. CFCs, carbon tetrachloride and HCFCs all contribute to depletion of the protective ozone layer in the upper atmosphere.