A: Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-476 (NULL Pointer Dereference) vulnerability exists in the code because it is comparing the `a` and `b` pointers for equality without first checking if either of them is NULL. If `a` or `b` is NULL, the comparison at line 4 will result in a NULL pointer dereference, which is CWE-476.