Which practice will prevent data loss in case of a computer malfunction?

You should back-up your computer every few weeks (or as often as needed).

As soon as you suspect that your computer (or any other backup-able device) is too slow or "glitch-y" you should back-up your device before you lose any important files or data.

PREVENTING MALFUNCTION:
→ Always set your computer on a hard & flat surface.
→ Avoid downloading programs from unknown or non-trusted sources.
→ Avoid using your computer in a hot area.
→ Consider installing a virus/malware protection software on your computer.