The answer is A. Kotowski cites four scholars who specialize in the study of volcanoes, whereas Ferguson cites only the encyclopedia.
The given thesis is arguing that Kotowski is more reliable than Ferguson because he backs up his statements with a greater number of more reliable sources. In choice A, Kotowski is not only using three more sources than Ferguson, but they're sources from reputable specialists, making them higher quality sources as well.
If you're still confused, you can use elimination to rule out the others:
B. B doesn't work because there's no mention of the rate of volcanic change in the thesis. Although the statement may be true, it's not evidence to support the writer's claim.
C. C doesn't work because there's no mention of the frequency of volcanic eruption in the thesis. Again, although the statement may be true, it doesn't necessarily support the writer's claim.
D. D doesn't work because there's no mention of the similarities between the two pieces. If the thesis is arguing that Kotowski's work is different from Ferguson's because it's more reliable, it'd be counterproductive for the writer to mention the similarities of the work. Therefore, this "evidence" has no relation to the thesis and would not support the author's thesis.
