Answer:
The verb in the sentence Sailing on the open sea was something that Jacob had never done before is used C) as a gerund.
Step-by-step explanation:
Verb variations ending in -ing can be either a gerund or a participle. In this sentence, sailing is functioning as a noun; therefore, it is considered a gerund. (Participles, on the other hand, function as participle clauses, adjectives or continuous aspects joined with the verb to be.)
The other options are not correct either: an appositive is a noun or noun phrase that provides information to further define another noun, and an infinitive is a form of the verb that does not have any ending and does not define any tense.