A 10-digit phone number field accepts digits only. Which example is invalid?

• A. 555-123-4567
• B. [phone]
• C. 5551234567
• D. 555123-4567X

Answer: (D)

The main idea is enforcing numeric input that results in exactly 10 digits. If a field accepts digits only, any non-digit character makes the input invalid, and the final value must be ten digits long.

Here, the examples that include formatting like hyphens or parentheses can often be normalized by removing those non-digit characters to yield ten digits, which some systems will accept as valid. The plain sequence of ten digits is obviously valid. The example that ends with a non-digit character X cannot be reduced to a 10-digit numeric value, so it violates the digits-only requirement.