The correct answer is: B. Risers plus one.
A flight of stairs is a series of steps that lead from one level to another. The number of treads in a flight is equal to the number of risers plus one. This is because each tread is one step, and each riser is the vertical space between two steps.
Option A is incorrect because the number of risers in a flight is not equal to the number of treads.
Option C is incorrect because the number of risers in a flight is not equal to the number of treads minus one.
Option D is incorrect because the number of treads in a flight is equal to the number of risers plus one.