A subject in grammar is the first part in a sentence about which the second part, the predicate, tells something.
Well, using my limited knowledge, "cleaving" means to chop, break in half, split, etc. Twain means twice. So, perhaps it's like "you've broke my heart in half", something along those lines? hopefully this helps you.
D. "this was an uneasy secret..."
The answer I think will be B