If blue light hits a red filter, what kind of light comes through the filter?
a. red
b. blue
c. purple
d. no light

It can be either C or B

Reasons it can be C: Red and Blue together(if I'm correct in art) is the combined color of two of the 3 primary colors to get a purple/violet color and if said filter is see through or just too dense for the light to even penetrate the said filter(in theory) but all in all purple is the answer with the two primary colors blue and red.

But also, it depends on what kind of filter it is, if the filter is like a screen filter then it will just come out in blue with the slightly different colors of again purple but in a darker tone then the actual eye can see.

Or it can be just C again cause the filter can be a film but that's a bit too far and to complex for right now so I believe it is B