In the context of this unit, what is a filibuster?

a) a large cattle drive that travels across more than one state or territory
b) a large number of people who all move to the same area at the same time
c) an action taken by members of the united states congress to stall a vote
d) an explorer who leads an unapproved military charge into a foreign country

I looked it up and thee answer that is most likely it is B

D. An explorer who leads an unapproved military charge into a foreign country.

This is because historically, the word comes from the Spanish word "filibustero", which meant somebody who invaded foreign countries. 

Had the context of the unit been about the American judicial system, the answer would've been C. an action taken by members of the United States Congress to stall a vote.