... which emerged in the mid-western region of Nigeria...'
i) What grammatical name is given
to this expression as it is used in the
passage?
ii) What is its function?

Noun clause

Explanation:

A noun clause or nominal clause is a dependent or subordinate clause that does the work of a noun in a sentence. It generally functions as an appositive, the subject or the object of a transitive verb, complement of a subject, object, and preposition.

I hope this helps you in any shape or form.