Which of the following is a type of software maintenance?

Software maintenance is the modification of a software product after delivery. The ISO/IEC 14764 (IEEE 14764) standard defines four standard categories:

1. Corrective maintenance - reactive changes that fix faults or defects found after delivery.

2. Adaptive maintenance - changes that keep the software usable in a changed or changing environment (new OS, hardware, platform, or regulations).

3. Perfective maintenance - changes that improve performance or maintainability, or add enhancements requested by users.

4. Preventive maintenance - changes that detect and correct latent faults before they become operational failures.

Corrective, adaptive, and perfective maintenance are all valid, recognised types of software maintenance. Since each of the listed options is individually correct, the complete answer is "All of the above."