The correct answer is C. UPS. A UPS, or uninterruptible power supply, provides temporary battery power during a power outage. This gives users or systems enough time to save work and shut down safely. Cloud storage, external drives, and file servers can store data, but they do not provide temporary power during an outage.
Exact Extract/Reference: The CompTIA Tech+ Study Guide defines a UPS as "a battery backup system" and states that during a power outage or brownout, it can supply power "for a limited amount of time and provide for a graceful shutdown." The guide also states that if power fails, UPS batteries can power the computer long enough for the administrator to safely power it down.