OPTION B
The number of times "PROCESS" is printed is equal to number of process created. Total Number of Processes = $2^{n}$, where n is number of fork system calls. So here$ n = 2, 2^{2} = 4$
fflush() is typically used for output stream only. Its purpose is to clear (or flush) the output buffer and move the buffered data to console (in caseof stdout) or disk (in case of file output stream).
The two fork() calls create $3$ child processes, and hence "PROCESS" will be executed $4$ times if we don't use fflush.
If we put a '\n' at end of printf or use fflush(stdout); only $1$ printf will be done.