Step 1: Chain the premises. All rats are cats, and all cats are chairs, so all rats are chairs. This is a valid chain (A-type + A-type gives A-type), so (D) follows.
Step 2: Some dogs are rats, and all rats are cats, so some dogs are cats, not all. So (A) fails.
Step 3: All cats are chairs does not reverse to all chairs are cats, so (B) and (C) fail.
Step 4: Only (D) follows, so the answer is option 4.