Answer:
The victim B is under attack.
Step-by-step explanation:
This scenerio is know as the TCP Half open scan. The attacker send to SYN packet to the victim just to know if a port is open.
A TCP Half Open Scan also referred to as SYN scanning determines if a port is open by performing the first half of a three-way handshake.
Attacker is trying to have TCP connection to a server through any port, this is done by sending a SYN (synchronization) packet, as if to initiate a three-way handshake, to every port on the server.