Which equation represents a line that is parallel to the y-axis?

(1) x = 5 (3) y = 5

(2) x = 5y (4) y = 5x

In a line parallel to y-axis, the x would have to be constant, if it's not constant (always the same) then it won't be parallel. Constant values of y could be y=1, y=2 ... etc, and we have this as an option: y=5, this is the correct answer.

(1) would be parallel to x axis and 2 and 4 are diagonal or sloping lines.