Apassive verb is:
a. a verb that allows the subject to receive the action.
b. a verb that indicates slow-moving action.
c. a verb that causes the subject to perform the action.
d. a verb that indicates lazy subjects.

It says in the text that millions or people had come from different parts of the country to go into Washington, D.C. to protest. So congregate means to gather around in a mass.

So the context clues would be “meet on the National Mall” “millions of people” “their voices heard”.

hope this helps