The boldface term that correctly indicates a gerund is B) VACATIONING is an important thing to do with your family.
The gerunds is a part of speech. Gerunds are words that are formed by a verb followed by ing, like the present participle. The important thing to remember is that gerunds are formed by verbs but they act as nouns. When you see an ing-form it can be a verb, the present participle complete form, it can also be a modifier or a gerund that is a verb used as a noun.