Some ores deep in the Earth are rich in iron particles. These particle

A.The iron particles were exposed to the atmosphere during the period when the ores were deposited. - This clearly supports the conclusion that the atmosphere contained less oxygen in the past than today.
On negating Option A i.e The iron particles were NOT exposed to the atmosphere during the period when the ores were deposited, the conclusion will fall

B. Ores were more vulnerable to the effects of corrosion in the past than they are today. - The argument is about iron particles exposure to oxygen and not ores exposure.
C. Ores are deposited at various depths in the Earth. - Irrelevant.
D. The oxygen level of the atmosphere remained constant during the period when the ores rich in iron particles were deposited. - Irrelevant.
E. Ores near the surface of the Earth are less rich in iron particles than are ores deep in the Earth.- Irrelevant

Conclusion: when the ores were deposited millions of years ago, the atmosphere must have contained less oxygen than it does today.

Premise: These particles are too vulnerable to the corrosive effects of oxygen to have accumulated under the oxygenated atmosphere of today.

Assumption + Premise = conclusion

A. The iron particles were exposed to the atmosphere during the period when the ores were deposited.
MUST BE TRUE, otherwise without contact argument breaks

B. Ores were more vulnerable to the effects of corrosion in the past than they are today.
Weak - breaks the argument

C. Ores are deposited at various depths in the Earth.
Let's think.. even if ores are deposited at 1 km & 2 km, how does that help? Irrelevant. At worst, weakening because ores are near surface and below so have deposited given the atm level in present & in past. no difference.

D. The oxygen level of the atmosphere remained constant during the period when the ores rich in iron particles were deposited.
weakening - goes against conclusion

E. Ores near the surface of the Earth are less rich in iron particles than are ores deep in the Earth.
does strengthen the argument, but even if ores near surface and ores in depth have the same iron content, the oxygen level could have been varied in the past. Moreover, we don't know about quality of "iron particles" that have accumulated, Does not need this to be true

It is one of the hardest question

Let's break down the argument and the answer choices more simply:

Argument:
Fact: Some deep-earth ores are rich in iron particles.
Problem: Iron particles are easily corroded by oxygen.
Conclusion: The atmosphere must have had less oxygen when these ores were deposited millions of years ago.
What does the argument assume?
For the argument to make sense, it assumes something that must be true for the conclusion to follow logically.

Answer Choices:
The iron particles were exposed to the atmosphere during the period when the ores were deposited.

If iron particles were not exposed to the atmosphere, the oxygen level wouldn’t matter. Therefore, the argument assumes they were exposed.
Ores were more vulnerable to the effects of corrosion in the past than they are today.

This does not relate to the oxygen levels in the atmosphere.
Ores are deposited at various depths in the Earth.

This is a general fact and doesn’t impact the argument about oxygen levels.
The oxygen level of the atmosphere remained constant during the period when the ores rich in iron particles were deposited.

If the oxygen level stayed constant, it would not support the argument that there was less oxygen in the past.
Ores near the surface of the Earth are less rich in iron particles than are ores deep in the Earth.

This doesn’t help explain the oxygen levels in the past.
Simplified Conclusion:
The correct choice is:

The iron particles were exposed to the atmosphere during the period when the ores were deposited.

This assumption is necessary because if the iron particles were not exposed to the atmosphere, the current oxygen levels wouldn't matter, making the argument about less oxygen in the past irrelevant.­