The word rate refers to:

cost

speed

percent

how fast or slow something is done

numerical order

Rate refers to the speed at which something is done, so your answer is D.

Answer;

how fast or slow something is done.

Explanation;

The term rate refers to how fast or slow something is done. For example; a higher rate shows how fast something is being done or is taking lace, lower rate shows how slow something is being done.

In other words it is a quantity measured with respect to another measured quantity; for instance; a rate of speed of 60 miles per hour.