Prob. $5\mathrm{.}18$

a$.$

Steps $1$ and $2$ are used to establish SA between the sender and the receiver.

b$.$

Steps $3$ and $4$ are used to establish a secret key between the sender and the receiver.

c$.$

Steps $5$ and $6$ are used to authenticate each other.

d$.$

All of the above.

e$.$

None of the above.