The correct answers are
[(b.) And in this flea our two bloods mingled be;]
Thou know'st that this cannot be said
and
[(d.) And pampered swells with one blood made of two;
And this, alas, is more than we would do.]
In this poem, the flea clearly symbolizes their love.
In the answer (b.), their bloods are mixed in this flea and this could metaphorically signify mixing their bodily fluids while engaging in intercourse.
In the answer (d.), it is clear that the speaker is denied intercourse with this woman and extends his argument from the sentence (b.) that their bloods mixed in the flee signify their physical union and that, despite what the society might suggest about her loss of virginity, there is nothing shameful about this act.
Look at this flea, and you'll understand that what you're denying me is very trivial. The flea sucked my blood first and then it sucked your blood. Now our bloods are mingled in the flea’s blood. This mixing of bloods is not a sin or anything to be ashamed of. The flea now grows big with a new life inside it. The little bloodsucking flea has achieved much more than what we as lovers have attained.
