Which ones are density curves for a continuous random variable

If you reflect over the y-axis, you're basically just folding the shape over the y-axis (vertical up/down line) which means if you were to look at the points, A-A' would share teh same y coordinate point but the x coordinate point would be inversed (become negative)

going through your choices
A. looks like its reflected over the x-axis
C looks like its reflected over the x-axis, then y-axis
D. looks like a translation from the 3rd quadrant to the 1st

B. is a reflection over the y-axis