The geologist is arguing that the scientists' view that fossil fuels formed from carbon deposits is wrong because of the presence of biomarkers in petroleum.
Answer choice A while it doesn't help the geologist's argument, does not weaken it because it says that "fossils have been discovered that are devoid of biomarkers". This means "some" have been found that do not have biomarkers. But we don't know why they do not have biomarkers, perhaps we can't detect them or they have been removed somehow. It does not attack that the presence of biomarkers in petroleum suggests that fossil fuels come from living organisms, as opposed to carbon deposits.
This differs from an answer choice such as D. By showing that certain bacteria can live deep inside the earth's crust, this suggests that the biomarkers come from these bacteria, rather than from a living organism resulting in fossil fuels. It shows that the fossil fuel is not necessarily related to the presence of biomarkers, there just happens to be some bacteria there.
I hope this helps. Please let me know if you have any other questions.