Which describes the crust where the yellow areas of the two circles are falling?

a.
spreading center

b.
divergent plate boundary

c.
convergent plate boundary

d.
transform plate boundary

This is a tricky one, Judging by what you asked me I looked up all of the meanings for those plate movements, Transform is when they move past eachother in opposite directions on their sides, Convergent plate movement is when two plates are moving towards eachother and one goes above the other one, pushing the other one down. Divergent is where magma push up the 2 plates pushing them apart but instead of sinking they are lifted up. The most logical answer I would say is Spreading center because when the two plates spread, both of the plates sink.  

Yes yes yes very good