System programs such as Compilers are designed so that they are

ANS:A - reenterable

In computing, a computer program or subroutine is called reentrant if it can be interrupted in the middle of its execution and then safely be called again ("re-entered") before its previous invocations complete execution.

The interruption could be caused by an internal action such as a jump or call, or by an external action such as a hardware interrupt or signal. Once the reentered invocation completes, the previous invocations will resume correct execution.