The scientist's hypothesis is: "the ferrous material which was contained in atmospheric dust, had promoted a great increase in the population of Antarctic algae such as diatoms."
Why? Scientists found that the ice-age atmosphere had contained unusually large amounts of ferrous material and very small amounts of carbon dioxide. We are also told that algae absorbs carbon dioxide from the atmosphere.
So we have a cause and effect hypothesis. Because of the ferrous material contained in the atmospheric dust, the Antarctic population of diatoms increased. So, how do we weaken a cause and effect argument? We show the cause without the effect.
That is actually what answer choice (D) is doing: "Sediment from the ocean floor near Antarctica reflects no increase, during the last ice age, in the rate at which the shells that diatoms leave when they die accumulated."
If there was an increase in the population in the diatoms, then, inevitably, there would be an increase in the shells left behind when these diatoms die. But, answer choice (D) states that this did not occur, i.e. it is unlikely that there was an actual increase in diatom population.
So, even though there was ferrous material in the atmosphere, the population of Antarctic algae such as diatoms did not increase--thereby weakening the scientist's hypothesis that the ferrous material caused an increase.
Hope that clears things up! Please let us know if you have any other questions.
