Final answer:
The correct verb to use is "ont". The verb "avoir" is used to express age.
Step-by-step explanation:
The correct verb to fill in the blank in the sentence "Ils ____ 21 ans" is ont.
In French, the verb "avoir" is used to express age. Since "Ils" refers to a plural subject, we use the conjugated form "ont". So the correct sentence is "Ils ont 21 ans".
Here, "ont" is the third person plural form of the verb "avoir" in the present tense.
Learn more about Using the correct verb to express age in French