Since 1868, approximately thirty-five shipwrecks have been recorded of

Dear Skywalker18,

I see that the brilliant sayantanc2k already gave you a good response. I will add a few thoughts.

First, about (B), I would say there are three categories
1) uncountable, for which we use "more" and "less" (less talk and more action)
2) countable, for which we use "more" and "fewer" (more women and fewer men)
3) numerals, for which we use "greater" and "less" (\(\pi\) is less than 4 and greater than 3)

The dash, technically know as an em-dash, is a piece of punctuation that indicates the continuation of a thought, even though there's, as it were, a break in the grammatical flow. This may be an example or a further clarification.
I went home early--I was coming down with the flu.
We also use a pair of dashes to set off either examples or an aside comment.
Early 18th century composers--Bach, Handel, Vivaldi--used the concerto as their primary orchestral form. (examples)
Reagan deregulated savings and loans banks--what a disastrous decision!--and thereby precipitated the S& L crises a few years later. (an aside comment)

Here, in (D), the phrase "fewer than have been recorded off the coast of Massachusetts" is an afterthought. It could be omitted, and the sentence would be grammatically whole. This extra information adds further significance to the facts stated in the sentence, and putting this comment after a dash gives it a kind of dramatic flair.

Much of the use of the dash concerns its rhetorical effect. I will remind you that the GMAT SC absolutely does not test punctuation. If you read sophisticated writing, you will develop intuition for the dash, but this is more than you need to know for the GMAT.

Does all this make sense?
Mike