Blood samples must be centrifuged soon after collection to (i)____ red blood cells from the serum or plasma, which then must be (ii)____ until assay, preferably in an ultralow rather than conventional frost-free freezer

clearly the second blank is the easier to work with.

Frozen

However,

the first part that we could call as X must be frozen thereafter a certain process

Now, blood cells from the serum or plasma,

You cannot synthesize something from a centrifuge, which is a forced rotation. It is the exact opposite process. So B is out

Dealing with C we do have no clue that we recover X from Y. We do have a process wherein input we have X and the output is Y. Recover is when you have a certain thing and you try to back to the previous status

For example, the vessel is broken into pieces and you try to recover somehow, gluing the various parts

So B must be the answer: we separate two components that together form the blood into two single elements

I hope now is more clear