Which sentence uses a transitive verb? a. the children climbed hills. b. several children like to climb. c. climbing is good exercise. d. a steep climb can be tiring.

The answer will have to be A because  transitive verb has two characteristics. First, it is an action verb, expressing a doable activity like kick, want, paint, write, eat, clean, etc. Second, it must have a direct object, something or someone who receives the action of the verb.

A transitive verb example would be: She played the piano.

I believe that the answer is B: The children climbed hills.

Would cloning of animals be allowed to prevent extinction?