The equation that makes the system have infinitely many solutions is one that is dependent on the given equation. The easiest way to make a dependent equation is to multiply the given equation by some constant.
Here, choice C has the original equation multiplied by 2, so it is dependent on the given equation.
x + y = -1 ⇒ 2(x + y) = 2(-1) ⇒ 2x + 2y = -2 . . . . matches choice C
