A and D would be the same, unless a is actually 4 + c.
I would go with D because four times a number.
Let's substitute a number for the letter c (variable).
We know it is multiplication, because of the key word: times.
Four times a number can be written as: "4 * c" or "4 x c" or "4(c)" or "(4)c" or "4c".
