Final answer:
A phrase is a group of words that does not contain both a subject and a verb, while a clause is a group of words that contains both a subject and a verb. In the sentence 'Who was ill?', the phrase 'who was ill' functions as a clause because it contains the subject 'who' and the verb 'was'. Therefore, the phrase 'who was ill' is a clause.
Step-by-step explanation:
A phrase is a group of words that does not contain both a subject and a verb, while a clause is a group of words that contains both a subject and a verb. In the sentence 'Who was ill?', the phrase 'who was ill' functions as a clause because it contains the subject 'who' and the verb 'was'. Therefore, the phrase 'who was ill' is a clause.