Which of the following is a valid data validation check for numeric input such as age?

• A. Must be a positive integer.
• B. Must be a string of letters.
• C. Must be a boolean value.
• D. Must include symbols.

Answer: (A)

For numeric input like age, you want to ensure the value is a whole number and makes sense in the real world. Requiring the input to be a positive integer achieves this by enforcing both the numeric type (it's a number) and the domain rule (ages are counted in whole years and are not negative). This rejects decimals, fractions, and negative numbers, and rejects non-numeric characters altogether. The other checks would not validate a numeric age: requiring a string of letters would fail the numeric requirement, a boolean value isn’t a numeric type, and needing symbols would also make valid ages invalid.