Final answer:
Defragmentation is the correct answer to ensure computer reliability, by organizing the data on the storage device efficiently. Overclocking and malware installation can harm a computer's reliability, while multitasking can affect performance but not necessarily reliability.
Step-by-step explanation:
To ensure a computer stays reliable, the correct answer is a) Defragmentation. Defragmentation is a process that reduces the amount of fragmentation in file systems. It does this by physically organizing the contents of the mass storage device used to store files into the smallest number of contiguous regions (fragments). It also attempts to create larger regions of free space using compaction to impede the return of fragmentation. Some systems automatically defragment storage periodically. Nevertheless, manual or scheduled defragmentation can still be beneficial for older mechanical hard drives.
Overclocking, choice b), can actually decrease the reliability of a computer by increasing the operating temperature and stress on components. Malware installation, choice c), is obviously detrimental to your computer's health and reliability. Choice d), Multitasking, does not inherently affect the reliability of the computer, but excessive multitasking can slow down a system and impact performance.