I see why (D), the correct answer, is not a strengthener (it's a weakener), but I do not see how (A) is a strengthener. I was pressed for time on this section, and so picked (A) after reading it and moved on. Was pretty pissed off about this one when I graded the section.
(A): For this answer to lend support, wouldn't it have to be true that no Ice Ages occurred prior to 800,000 years ago? Because the stimulus neither explicitly states nor implies this. It states that the Earth has undergone a regular sequence of Ice ages beginning 800,000 years ago. All that can be gleaned from this is that there was not an ice age prior to 800,000 years ago that occurred "x" number of years prior to 800,000 years ago, "x" being the amount of time, the regular interval, between Ice Ages in the sequence that began 800,000 years ago; or, alternatively, that no two or more Ice Ages prior to 800,000 years ago occurred "x" years apart, and the most recent one of those occurred "x" years apart from the one 800,000 years ago. So the language in the argument leaves open the possibility of an Ice Age occurring prior to 800,000 years ago, or even a regular sequence of them occurring at some point prior to 800,000 years ago that does not form a regular progression with the sequence beginning 800,000 years ago. Thus, no cosmic dust prior to 800,000 years ago does not support the climatologist's hypothesis, because there could have been Ice Ages prior to 800,000 years ago, and if there were, then they would not have been caused by cosmic dust.