Answer:
The answer to the question: In which sentence does the gerund "firefighting" function as a subject, would be: B: Firefighting is definitely a dangerous job.
Explanation:
Although a gerund looks like a verb, in its present participle form, the enormous difference is that the gerund will always function as a noun. Sometimes, this noun function can be as a subject, when it initiates the sentence, it can be used as a complement of the verb "to be", it can come after prepositions, after phrasal verbs, after expressions, and even after other nouns. However, in this case, the question requests that the gerund be used as a subject, which means, it has to initiate the sentence. The one option that gives that is option B. That is why it is the right answer.
