A: Root Cause Analysis: Looking at the given vulnerable code line and its context, the CWE-787 (Out-of-bounds write) vulnerability exists in the code because the `_stream` may not be properly initialized. This can happen if `_stream` is NULL or if the `send` method is not properly initialized. In this case, an Out-of-bounds write vulnerability can occur when `_stream->send` is called with invalid parameters, which is CWE-787.