Why is a battery on my motherboard?

During a routing cleaning of the inside and outside of a desktop computer, you notice that a small battery is mounted on the motherboard. What is this battery used for?

A) It keeps the system memory intact when the system is in “hibernate” mode

B) It powers the internal system cooling fans

C) It’s an on-board uninterruptable power supply (UPS)

D) It maintains the state of the BIOS settings