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A critical region is

1. One which is enclosed by a pair of $P$ and $V$ operations on semaphores.
2. A program segment that has not been proved bug-free.
3. A program segment that often causes unexpected system crashes.
4. A program segment where shared resources are accessed.
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A critical region is a program segment where shared resources are accessed.. that's why we synchronize in the critical section. That is (D).

PS: It is not necessary that we must use semaphore for critical section access (any other mechanism for mutual exclusion can also be used) and neither do sections enclosed by $P$ and $V$ operations are called critical sections.
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