Answer:
The adverb clause is "unless she is ill" and it modifies the verb "comes".
Explanation:
An adverb clause functions as an adverb, modifying an adjective, a verb, or another adjective. It can indicate manner, place, time, cause etc., and it must present a subordinating conjunction.
In the structure "Mary comes to school unless she is ill", the clause "unless she is ill" - introduced by the subordinating conjunction "unless" - is an adverb clause of condition, since it provides the circumstance under which the action in the main clause will or won't take place. Because it concerns the action, it modifies the word that expresses the action, which is the verb "comes".
