Final answer:
In the equation D = rt, 'D' stands for distance, which is the length covered when an object moves at a certain rate (speed) for a given time.
Step-by-step explanation:
In the equation D = rt, where 'r' represents the rate or speed, and 't' represents time, the letter 'D' stands for distance. This is a basic formula used in physics and mathematics to relate these three fundamental concepts of motion, where rate is the speed of movement, time is the duration of travel, and distance is the length covered during the movement. The correct answer to what 'D' stands for in the equation D = rt is (a) Distance.