Fred is the network administrator for his company. Fred is testing an internal switch. From an external IP address, Fred wants to try and trick this switch into thinking it already has established a…

The RST flag terminates existing connections rather than simulating one, and 'SIN' is not a valid TCP control flag, making this option technically incoherent.

The SYN flag signals the start of a new three-way handshake, which initiates a fresh connection rather than impersonating an already-established session.

A packet with ACK set to zero resembles the initial SYN of a new connection, not an established session, and spoofing the switch's own IP as the source address would not represent an external host attempting the trick.

In a standard TCP connection, the ACK flag is set on every packet exchanged after the initial SYN handshake, signaling that both sides have an established session. By sending a packet with the ACK bit set and his own external source address, Fred presents the switch with a packet that resembles mid-session traffic rather than a new connection attempt. Some switches and stateful devices that do not fully validate session state can be tricked into accepting this as part of an existing flow.