Explanation:

- To find the arithmetic mean, you add all the numbers together and then divide by the count of numbers.

- Given numbers: 23, 17, 20, 19, 21.

- Step 1: Add them up: 23 + 17 + 20 + 19 + 21 = 100.

- Step 2: Count the number of values: There are 5 numbers.

- Step 3: Divide the total by the number of values: 100 ÷ 5 = 20.

- Option 1: 20 This is the correct answer.

- Option 2: 19

- Option 3: 23

- Option 4: 21