Answer:
One.
Explanation:
When two or more domains (trees) not sharing a contiguous namespace are joined together, a forest would be formed.
This ultimately implies that, a forest is formed by joining two or more independent domains having non-contiguous namespaces are joined together.
For instance, joining brainly1.com and brainly2.com would result in a forest i.e a single Active Directory environment.
In this scenario, Mary is working on joining of a new domain tree to an existing forest. Before starting the process, she should have at least one domains present. This single domain is typically known as the root domain.
Hence, joining a new domain tree to an existing forest requires that a user must have at least one domains present.
