Answer : unless she is ill
The adverb clause of the sentence above is "unless she is ill". It can be recalled that an adverb clause is composed of a subject and connected by a subordinating conjunction. In this case, the subordinating conjunction is unless and this conjunction keeps the clause from expressing a complete thought. The adverb clause "unless she is ill" also answers the wh question "When will Mary comes to school?", making it the correct adverb phrase.
