(Look at the pic) what is FC?

The element on the position (1,1) in the product matrix FC is a scalar product of the first row in F and the first column in C.

Therefore the element in the position (1,1) in FC will be a scalar product of (-2,0) and (12,1):

The only matrix that has a value of -24 in the position (1,1) is matrix B.

Therefore, the answer is matrix B.

i assume that by 5 you mean 5 gallons.

(20)(0.25) = 15

removing 15 gallons reduces the tank from 20 to 5.