Prob. $8\mathrm{.}2$

a$.$

A circuit gateway is placed at the transport layer. An encrypted IP packet is decrypted before it is passed up to the transport layer, and the encrypted TCP packet encapsulated in the IP packet can then be decrypted by the the circuit gateway.

b$.$

The circuit gateway can therefore decrypt and extract the TCP header. Thus, circuit gateway can relay encrypted packets.

c$.$

None of the above.

d$.$

Circuit gateways can only relay encrypted IP packets whose TCP payloads were not encrypted at the transport layer.

e$.$

Both a and b$.$