Which statement best describes 'reject invalid data early' in input validation?

• A. Reject invalid data only after processing has begun
• B. Accept invalid data initially and correct later
• C. Validate inputs at the end of the pipeline
• D. Validate inputs early and reject invalid data promptly with safe error messages

Answer: (D)

Fail fast by validating inputs at the earliest point possible and rejecting anything that doesn’t meet the requirements. This approach stops bad data from moving through the system, reducing the risk of data corruption, wasted work, and security vulnerabilities. Providing safe error messages means you report the problem clearly without leaking internal details, helping users or downstream systems fix the issue quickly. The described approach—checking early and rejecting invalid data promptly with safe messages—best captures the practice.

Why the other ideas don’t fit: waiting to reject until processing has begun lets invalid data influence initial decisions; initially accepting invalid data and correcting later risks inconsistent state and possible downstream errors; validating at the end of the pipeline allows bad data to circulate and potentially cause security or reliability problems.