Answer is (A)
A relation is given (Registration_Num, UID, BankAccount_Num, Name, Hostel_Room).
Now, Registration_Num is unique for each student. So with this, we can identify each student. Hence, this can be the primary key.
UID: It's an identification number for a person in a country. $($Say you're in India and your UID is $0243.$ Someone in Pakistan may also have the same UID as $0243).$ So, if all students are from India (that is, the same country) then their UID will be different and then UID will be a Candidate key.
If S is a super key then S $\cup$ UID will be a Super key. e.g. R(A, B, C, D), If AB is a superkey then ABC, ABCD are also superkey.
BankAccount_Num is not a candidate key, because a student can have multiple accounts or joint accounts. We can not identify each student uniquely with BankAccount_Num.