If T and R are tread and rise respectively of a stair, then A. 2R + T = 60 B. R + 2T = 60 C. 2R + T = 30 D. R + 2T = 30

The correct answer is $\boxed{\text{A. }2R + T = 60}$.

The formula for the total rise and run of a stair is $R + T = 25$. The rise is the vertical distance between two consecutive treads, and the run is the horizontal distance between the front edge of one tread and the front edge of the next tread.

In this case, we are given that $T$ is the tread and $R$ is the rise. We can substitute these values into the formula to get $R + T = 25$. Simplifying the equation, we get $2R + T = 60$.

Option $\boxed{\text{B. }R + 2T = 60}$ is incorrect because it does not take into account the fact that the rise is the vertical distance between two consecutive treads. Option $\boxed{\text{C. }2R + T = 30}$ is incorrect because it does not take into account the fact that the run is the horizontal distance between the front edge of one tread and the front edge of the next tread. Option $\boxed{\text{D. }R + 2T = 30}$ is incorrect because it does not take into account the fact that the rise and run are related by the formula $R + T = 25$.