The popular view is that Ozone layer’s depletion is real, as certain a

Here's the official answer and explanation:

Understanding the passage

Conclusion:
The attribution of depletion of Ozone layer to man-made chemicals is not true.

Premises:
1. Mount Pinatubo in the Philippines spewed forth more than a thousand times the amount of ozone-depleting chemicals in one volcanic eruption than all the fluorocarbons manufactured by wicked, diabolical and insensitive corporations in history
2. Mankind can't possibly equal the output of even one eruption from Pinatubo, much less 4 billion years' worth of them

In this case, premises 1 and 2 are set of dependent information pieces and comprise a single reason used by the author to arrive at the conclusion. In essence, the reason is:

Ozone depleting chemicals produced by Mount Pinatubo is much greater (probably billion of times) than ozone depleting chemicals (fluorocarbons) produced by mankind.
This reason is used to signal that man-made chemicals can only play an insignificant role in the depletion of Ozone layer.
Therefore, (Conclusion) Man-made chemicals can’t be held responsible for ozone layer depletion.

Prethinking

As we can see, the argument is talking only in terms of the amount of ODC ( Ozone Depleting Chemicals), not in terms of their impact on the ozone layer. This is a missing link that is needed to establish the correctness of the argument.
So, a statement like “ODCs generated by volcanic eruptions have the same effect on the Ozone as ODCs generated by mankind”, would complete the argument.
However, this is not a must-be-true statement. Why? Because we know that the quantity of volcanic ODCs is probably billions of times greater than man-made ODCs, therefore, even if volcanic ODCs have much less effect than man-made ODCs, our conclusion would still hold.

Therefore, our only requirement can be that volcanic ODCs have at least some effect on the Ozone layer.
However, why would ODCs be called so if they don’t have any effect on Ozone layer. So, they have to have some impact. This leads me to think that there are only two situations in which the argument won't hold:

1. Volcanic ODCs don’t reach Ozone layer due to some reason (hypothetical) –
a. Probably because due to their high temperature. they react with some chemical as soon as they reach atmosphere and this reaction snaps their ability to deplete Ozone layer
b. Their chemical or physical properties is different from man-made ODCs and these properties prevent them from reaching Ozone layer

2. Volcanic ODCs can’t deplete unless the depletion has been started by man-made ODCs. Suppose, Ozone layer exists in a very stable state – this state cannot be affected by volcanic ODCs. However, once man-made ODCs can start reactions with this stable state of Ozone and deplete it, this makes the Ozone layer reactive even to Volcanic ODCs.

In both the situations above, our conclusion won’t hold. Therefore, negation of either of the two statements could be a valid assumption and therefore, answer to our question.

Analysis of Answer Choices

A. It would take mankind more than 4 billion years to destroy Ozone. – So what? We are concerned with the cause of depletion, not the time-frame. Incorrect.

B. Each molecule of ozone depleting chemical released during an eruption of Mount Pinatubo destroys the same quantity of ozone as a molecule of fluorocarbons. – As we analyzed in the pre-thinking, this is not required. Even if a molecule of volcanic chemical destroys much less ozone than a man-made chemical, the conclusion will still hold. Therefore, this is not a must be true statement. Thus, Incorrect.

C. The amount of ozone-depleting chemicals released during a single eruption in Mount Pinatubo is much higher than the quantity of fluorocarbons produced by the companies – This can be inferred from the passage. Therefore, this presents no new information and thus, cannot be an assumption. Incorrect.

D. The molecular structure of ozone-depleting chemicals released during a volcanic eruption does not prevent them from reaching the stratosphere, the layer of the atmosphere where the ozone layer resides. – This is in line with our prethinking. If these volcanic ODCs don’t reach ozone layer, then the reasoning of the argument breaks down. Therefore, this is a must-be-true statement for the argument to hold. Correct.

E. The rate at which an ozone depleting chemical, whether man-made or released in a volcanic eruption, is released is not more important in the destruction of ozone layer than the quantity of chemicals released. – Given this statement is in a negative form “not more”. Let’s negate it and see if the conclusion breaks down. We can see that even if rate is more important than the quantity, it doesn’t impact the conclusion, since we don’t know whether slow rate depletes Ozone layer more or fast rate depletes more. Therefore, Incorrect.

Therefore, the answer choice is Option D.

Hope this helps

-Chiranjeev Singh