The correct answer is A. mean.

Explanation:
- Mean: This is the arithmetic average of a set of numbers. It is calculated by adding all the numbers together and then dividing by the total count of numbers.

- Median: This is the middle value of a data set when it is ordered from least to greatest. If there is an even number of observations, the median is the average of the two middle numbers.

- Mode: This is the value that appears most frequently in a data set.

In this case, when you refer to the "arithmetic average," you are describing the mean.