FortiGate permits an SD-WAN zone to contain a single member. Therefore, removing one member from a zone that originally contains two members does not violate the minimum membership requirement. The important dependency check is whether the member is still referenced by SD-WAN rules, performance SLAs, health checks, or other configuration objects.
The question explicitly states that the member is no longer used by health checks or SD-WAN rules, so FortiGate can remove it without requiring another operation. FortiGate does not automatically transfer a deleted member into the default SD-WAN zone. Likewise, removing an SD-WAN member does not mean that unrelated configured static-route objects must automatically be deleted. Option B incorrectly assumes that every user-defined zone requires two members. Therefore, FortiGate accepts the deletion with no further action.