Explanation:
a) is
b) are
c) am
d) was
The correct answer is: b) are
Subject “They” is plural, so it must take the plural verb “are”.
Correct sentence: They are going to school.
Other options:
is → used with singular subjects (He is, She is).
am → used only with “I” (I am).
was → past tense, not correct here since sentence is present continuous.