This is both irrelevant and descriptively inaccurate. The argument isn’t concerned with what the astronomers believed in the past; only their current views on this issue are referenced.

This describes how the astronomer chooses to accept Swiderski and Terrada’s position over Tagar’s without providing any justification for doing so. Just in case you're misunderstanding the grammar of (B) -- the two competing sets of authorities are Tagar (one set) and Swiderski and Terrada (the other set). The author assumes Swiderski and Terrada are correct over Tagar.

The astronomer never appeals to the number of experts who endorse a theory as evidence of its accuracy. We don’t know how many other authorities share either of the views discussed.

The astronomer only appeals to one view—Swiderski and Terrada’s—and assumes that it disproves Tagar’s. The flaw is failing to explain why it should be preferred over Tagar’s.

The argument does the opposite of this, assuming that Swiderski and Terrada’s theory disproves Tagar’s. In doing so, it treats one opinion as more justified than another.