Answer:
(a) PROPORTION.
Explanation:
A relative frequency histogram has bars whose height is equal to either the proportion of cases that are between the upper and lower bounds of the bar.
The relative frequency in a relative frequency histogram refers to PROPORTION.
A relative frequency histogram is a graph that has the same shape and the same horizontal scale as the corresponding frequency histogram. The difference is that the vertical scale measures the relative frequencies (percentages or proportions).
Hence, option (a) is right option.