How do you make a make a magma more silica-rich?

How come you don't know the answer?

I know nothing about this subject but I would guess Option "A" based purely on logic.

To make something "more" rich in something, you generally have to Add something to it, usually the thing you're trying to make it more rich in...

Sea Water is mostly water and salt, so I don't see how that could add much silica to anything. And Partial Melting of Crustal rock isn't even an answer. The pretty much leaves Option A.

But remember, I now nothing about this subject.

I would say to genetically modify bacteria to eat the magma and secrete a blend of the two compounds.

Plant vegetation may be an avenue to explore also.

All three a, b and c contain varying amounts of silica.

Sand is high in silica.