You forgot to add the options, however, I will tell you the concept of the problem and you can then choose the correct choice from the ones you have.
First, the slope of a vertical line is 1/infinity which is undefined.
Second, for a vertical line, you can notice that the value of the x is constant along the line.
Third, the y-axis has an equation x=0. Any line parallel to the y-axis will be located at a distance d from the y (d can be positive or negative depending on whether the line is to the left or to the right of the y-axis). Therefore, the equation of any line parallel to the y-axis would be x=0+d
Based on the above, the correct choice would be the equation in the form of:
x = constant
