What does input validation ensure in a program?

Input validation is a crucial process within programming that ensures the data entered into a system meets predefined specifications or criteria before it is processed. This can involve checking for the right type of data, ensuring data falls within acceptable ranges, or confirming that mandatory fields are completed.

By enforcing these criteria, input validation helps maintain the integrity and reliability of the data the program operates on. This is especially important for security, as it can prevent harmful data from entering the system, which can lead to errors or vulnerabilities such as injection attacks. Therefore, the ability to ensure that the input meets specified criteria directly impacts the functioning and safety of the software application.

The other options present different concepts: running faster relates more to performance optimization, while always correct implies a level of certainty that isn't realistic in programming, as input may still be logically incorrect despite passing validation. User-friendliness pertains to the design and experience of the user interface, which is a separate concern from the technical requirements of input validation.