What should an administrator enable to protect a vRealize Operations cluster against a single node f

What should an administrator enable to protect a vRealize Operations cluster against a single node failure?

A. application remote collector
B. remote data collector

C. replica node

D. additional master node

Reference: https://docs.vmware.com/en/vRealize-Operations-Manager/7.5/com.vmware.vcom.core.doc/GUID-071E3259-625A-437B-AB34-E6A58B87C65B.html