Question

The ceremony in which men became vassals to their lord was called:

Answer 1

I believe it is called homage. homage is a formal public acknowledgment of feudal allegiance.
ex: "a man doing homage to his personal lord."

Answer 2

The correct answer is:  

Homage.  

Homage was a ceremony developed in the Middle Ages. In this tradition a feudal tenant or vassal promised reverence and obedience to his feudal lord, getting in exchange the symbolic title to his new investiture. It was a symbolic recognition to the lord, claiming that the vassal was his.