which best identifies the underlined clause? the egyptologist, who had worked with the business person, was convinced that king tut's tomb was in the valley of the kings.
a.
adjective clause
b.
adverb clause
c.
independent clause
d.
noun clause

So the clause is "who had worked with the business person."
You can ask "why did they put the clause in here" or "what's the purpose of this clause?"
Well it describes the Egyptologist, and Egyptologist is a noun.
And adjectives describe nouns.
Your answer is A. adjective clause.

The underlined clause is most probably "who had worked with the business person". It tells more information about the noun the Egyptologist. And since it describes a noun, it is therefore an adjective.
A. Adjective Clause
Hope this helps. 

Idon’t know to be honest