Answer:
D. Mainframe computers
Explanation:
Mainframes computers are those generally known for their large size, amount of storage, processing power and high level of reliability. They are primarily used by large organizations for mission-critical applications requiring high volumes of data processing. All mainframes have the ability to run (or host) multiple operating systems and can add or hot swap system capacity without disruption. Moreover, mainframe computers can replace dozens or even hundreds of smaller servers.
