Question

The man_____you should marry is the one ______ gave me the portfolio.
A. who:who
B. Whome:whom
C. whom:who
D. who:whom

Answer 1

"who" is used in the subject position in a sentence.
"whom" is used in the object position.

Take out the #1 dependent clause "___ you should marry"

Then you have "the man is the one" (main clause) "____ gave me the portfolio ( #2 dependent clause)

Now, look at the clauses. #2 is easy....
#1 has to be rearranged. "you should marry _____"

If you still have problems figuring this out, please repost.