Ex: She was one of the teachers who was selected.
a) was

b) were
_________________ Comment Who refers back to the noun phrase
one of the teachers, the head of which is the singular noun
one, which is why the verb is singular. You can test this:
Ex: One (of the teachers) was selected.
Additionally, relative pronouns (e.g.,
who) refer back to the closest noun. In our example, the plural noun
teachers is the closest noun from a linear perspective; i.e., it's next to
who, but syntax doesn't work in linear terms. The prepositional phrase
of the teachers is embedded or hidden inside (
[...
]) the noun phrase
one of the teachers,
... [one [of the teachers]] who was ...
Structurally,
who is closer to the noun
one (
[...
]) than it is to the noun
teachers, and the reason the verb is singular
was. The verb cannot be plural as
who cannot see inside the prepositional phrase (
[...
]).