Which does the sentence contain?
read the sentence.
to the best of our knowledge, he decided that his major
would be acting
a conjunction
a gerund
an infinitive
a participle

A gerund

Explanation:

A gerund is a word that looks like a verb but that does not act as one, and that, instead, acts as a noun in a sentence. It is formed with a verb root plus the ending “ing,” like “winning” and “thinking.” The sentence provided contains a gerund, namely, “acting” which in this case functions as a predicate noun, completing the linking verb “be” and renaming the noun “his major.”