A surety's obligation is strictly bound by the terms he actually agreed to, since a guarantee is a promise given on the faith of one particular set of facts about the principal debtor's obligation. Testing each option against this rule against unilateral rewriting of the surety's bargain, rather than jumping straight to a section number, gives a reliable way to find the right answer.
Because Bank A and X changed the interest rate and repayment period between themselves without informing or obtaining Y's consent, the risk Y actually agreed to bear was altered unilaterally, and the law responds by discharging Y only to the extent of that alteration, not entirely and not leaving him fully bound either.
So the correct answer is Y is partially discharged from liability because Bank A's modification increased the risk to Y without his consent.