When identifying the noun form of a verb, look for the base word or form. "Leaving" functions as a gerund (verb form used as a noun) but is not the base noun form.
The noun form of the verb "leave" is "leave" itself. "Leaving" is the gerund form (verb + ing), "lively" is an adjective, and "left" is the past tense of the verb. Therefore, the noun form of "leave" is simply "leave."