Choice "C" is correct. The hot site is not just sitting there waiting for a particular customer to declare a disaster. Disaster recovery service providers normally have an extensive amount of floor space and an extensive amount of equipment, but nowhere near enough if all (or even a significant number of similar customers) declared a disaster at the same time. How much is needed is determined on a probabilistic basis; to the disaster recovery services provider, geographic and industry diversification of customers is extremely important. Nonetheless, service restoration would be provided in hours.
Choice "b" is incorrect. Cold sites have fewer resources than hot sites and would not fully respond in few hours.
Choice "a" is incorrect. The term warm site refers to blended capabilities. Warm sites would not respond as rapidly as hot sites.
Choice "d" is incorrect. An in-house solution would use available capacity for critical functions. Hot sites typically involve external providers and off premises facilities.