Final answer:
The agreement that enables mutual site use between two organizations during emergencies is called a Reciprocal agreement. It involves a mutual understanding rather than one party providing the facility. This setup is cost-effective for both parties involved. So option ( D) is correct.
Step-by-step explanation:
The type of agreement that enables two organizations to provide mutual use of their sites in the event of an emergency is known as a Reciprocal agreement.
In disaster recovery and business continuity planning, such an agreement involves several arrangements. A Backup site agreement could generally refer to any arrangement to use a backup site, but it doesn't imply mutuality. Warm site and Hot-site agreements refer to specific kinds of backup sites where some level of equipment and data readiness is maintained; however, they alone do not imply an inter-organizational mutual support arrangement, as does a reciprocal agreement.
A reciprocal agreement is particularly cost-effective since it allows organizations to support each other without necessarily investing in dedicated disaster recovery infrastructure on an ongoing basis.