Which of the following statements about the Two Phase Locking (2PL) protocol…

Answer: The true statements are the ones that assert the protocol guarantees serializable schedules, that once a lock is released no new locks can be obtained thereafter, and that deadlocks are possible.

Explanation:

• Two-Phase Locking guarantees conflict-serializability by dividing each transaction into a growing phase (acquiring locks) and a shrinking phase (releasing locks). Because no new locks are acquired after the first release, the execution is conflict-equivalent to some serial order.

• The description that a transaction locks an item just before each operation and releases it immediately after each operation is not 2PL. Immediate release after each operation would allow acquiring locks after releases and thus violate the two-phase requirement.

• By definition, once a transaction releases any lock it has entered the shrinking phase and cannot obtain any new locks. This is the core 2PL constraint.

• Deadlocks can occur under 2PL because transactions may wait for locks held by others, creating cycles (for example, one transaction holds X and waits for Y while another holds Y and waits for X). 2PL ensures serializability but does not eliminate deadlocks.

Summary: The correct statements are the ones asserting that 2PL permits only serializable schedules; that no new locks can be obtained after a lock is released within a transaction; and that deadlocks are possible. The statement about locking and immediately releasing around every operation is incorrect.