Final answer:
The correct completion for the sentence "Neither of the girls ______ tired." is "is," because the subject 'neither of the girls' requires a singular verb.
Step-by-step explanation:
The verb that correctly completes the sentence "Neither of the girls ______ tired." based on the sentence diagram and subject-verb agreement rules is "is." The sentence subject "neither of the girls" takes a singular verb because 'neither' refers to neither one nor the other in a pair, meaning it is singular. Therefore, the correct sentence is "Neither of the girls is tired."
Additionally, when compound subjects are joined by 'or' or 'nor', the verb agrees with the word closest to it, further supporting the choice of a singular verb 'is' in this context.