How to determine regressive tax
You're correct it is graph A. Regressive tax means that the lower income you have the higher %tax you have. If you imagine at the lowest level of income, you still have a flat tax(The intercept).

Since the slope is constant the %tax component is the same for all income levels.

Therefore this flat tax component is the part you need to look at. The flat tax represents a larger proportion for lower income earners. As a whole (%tax + Flat tax) the tax is then a higher % for lower income earners, hence regressive taxation.