Answer: Third option is correct.
Explanation:
Since we have given that
If there is a storm, then there is thunder.
If there is thunder, Li's dog is under her bed.
Since both the statements are positive.
There is a storm ⇒ There is thunder
There is thunder ⇒ Li's dog is under her bed.
It means There is a storm ⇒ Li's dog is under her bed.
Hence, third option is correct.