Explanation:

- The average age of the father and mother is 40 years. This means their combined age is 40 * 2 = 80 years.

- The average age of the father, mother, and daughter is 30 years. This means their combined age is 30 * 3 = 90 years.

- To find the daughter's age, subtract the combined age of the father and mother (80 years) from the total combined age (90 years).

- 90 - 80 = 10 years.

- The age of the daughter is therefore 10 years.

Option 3: 10 is the correct answer.