By definition, it happens value of MBS issued < Value of underlying mortgate pool. But none of the answer is close to it. CCA and excess spread are correct. There is one option which is phrased other way i.e., Value of MBS issued > value of underlying mortgage pool which is wrong.
And regarding the liquidity crisis, it seems you are correct. But I donot remember reading repo, rolling over etc.. in that question. How are we supposed to know that it comes from current issue and not from credit risk. CFA is far better in that way. Atleast they ask question section wise.