The author assumes that the fact the approach described hurts our ability to survive shows that it’s not the surest way to increase overall correctness. (A) captures the author’s assumed connection between hurting survival and the surest way to increase overall correctness.

The statisticians’ rule does not allow for accepting new beliefs. So the author doesn’t overlook this possibility.

The author never takes a position on whether larger sets of beliefs are more or less correct than smaller sets. The possibility in (C), if true, would not undermine the author’s reasoning.

The author never takes a position on what one “should” do. The argument is simply about whether a particular approach is the surest way to increasing overall correctness of beliefs. What one should believe or not believe doesn’t affect the reasoning.

The argument concerns the overall correctness of a set of beliefs and the need for many beliefs to survive. Perhaps some beliefs can be false within a set of many beliefs; this doesn’t undermine the reasoning.