Official Explanation
The jump from the third sentence to the fourth is interesting in this argument. The third states: math studies "numbers and shapes and functions and their more abstruse extensions" and that the implications for science aren't decisively important.
The fourth sentence states: math is not a natural science. To make this jump, we need some assumption that tells us what the criterion would be for discipline to be a "natural science," and why a field that studies "numbers and shapes and functions and their more abstruse extensions" would not qualify.
The whole issue of whether one discipline "serves" another is a big distraction to the main argument.
(B) is the credited answer. One can count physical things, but a pure number itself is not a sense object. There are physical objects in the shape of, say, a triangle, but the triangle itself is not a sense object. Certainly functions and "their more abstruse extensions" are not sense objects. If focus on tangible sense objects is the criterion that qualifies a discipline as a natural science, it's easy to see why mathematics does not meet this criterion. This completely explains the argument.
(BTW, the vast majority of mathematicians and scientists would wholeheartedly agree both with the conclusion as well as with the credited answer here.)
(A) & (D) & (E) all focus on the distractor issue. Whether or not math "serves" another discipline does not, in and of itself, speak to whether it qualifies as a natural science. In addition, (D) introduces the idea of "rank," implying that the natural sciences occupy a certain "rank" and that mathematics, in serving them, would be below this "rank"—none of this finds any support in the prompt.
(C) is anecdotal evidence: what is true for just one individual or for a handful of individuals does not reflect on the nature of the discipline of mathematics itself. (BTW, Einstein really did struggle with mathematics!)