To ensure the serializability, we need two-phase locking protocol (2PL).
2PL has four cases:
- Simple or Basic
- Conservative
- Strict
- Rigorous
In Conservative 2-PL we need to lock all the data items which the transaction required before the transaction starts. And for its conservative nature, this protocol is deadlock free but starvation can occur.
So, the ans. is (D) Should acquire all the locks only at beginning of transaction.