Answer : 2. "Somniloquist"
Explanation :

The appropriate one-word substitution for 'A person who talks in sleep' is:

B) Somniloquist

Here's a brief explanation of the options:

A) Somnambulist: A person who walks in their sleep.

B) Somniloquist: A person who talks in their sleep.

C) Insomniac: A person who has difficulty sleeping.

D) Somnolent: Feeling sleepy or drowsy.

So, Somniloquist is the correct term for someone who talks while asleep.