As a multiple choice question, this is very difficult to answer. Agricultural science concentrates on the raising of plants and animals for human use, so we must accept choice B, plants, as a correct choice. However we have that devilish "All of the above" option to consider too, so we have to examine the other choices carefully.
I would say that choice A, atmosphere, is a correct choice as well; those involved in agriculture are deeply dependent on both weather and climate, so an understanding of the fundamentals of how the atmosphere works would be a very useful thing to know.
This leaves us with choice C, fossils, to evaluate. This seems to be a less obvious choice. However, all agricultural science majors are required to study soil science and most study geology as well, and this would include fossils. While the fossils themselves do not have much to do with farming, understanding the soil and the rocks that created it certainly does, and fossils are an indicator of the history and structure of the rocks in an area. Therefore fossils could be considered to be part of one's studies in agriculture.