The answer is D. "What are the harmful effects of cigarettes?"
An intransitive verb is a type of a verb which doesn't need a direct object after it. As you can see, sentences A, B, and C have direct objects following a verb (A: balance and agility; B: sick or injured skiers; C: snowboards to the ski slopes) which means they all have transitive verbs.
Therefore, the correct answer is D. Winter storms arrive suddenly in the mountains.
The Civil War was devastating. Was is our linking verb.
