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 the 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 the 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 the surface and ores in-depth have the same iron content, the oxygen level could have varied in the past. Moreover, we don't know about the quality of "iron particles" that have accumulated, Does not need this to be true

Answer: A