Answer: D. firefighting is definitely a dangerous job.
Explanation: The subject of a sentences is who performs the action of the verb, it can be a person, an animal, an object or a group, also the subject is usually formed by nouns. From the given sentences, in the first three options (A, B, C) we can see that the word "firefighting" acts like the direct object of each sentence, because it answers to the question "what" ("what does a successful firefighter enjoys?"), the sentence in which "firefighting" is the subject is option D.
