How does a warm front form?
a.

warm air becomes caught between cold air masses.
b.

two air masses meet and stay separated.
c.

warm air moves over cold air and replaces it.
d.

cold air moves under warm air and pushes it up.

A warm front occurs when a cold air mass retreats and is replaced by a warm air

I hope that's help !

A weather front is the boundary between two air masses

A warm front forms when arm air moves over cold air and replaces it. Correct C

The answer comes from the definition of warm front itself: A warm front is defined as the transition zone where a warm air mass is replacing a cold air mass.