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.