Final answer:
The sentence that has an inappropriate shift in number is: 'If my friend calls, tell them I'm at my violin lesson.' In this sentence, the subject of the sentence, 'my friend,' is singular, but the pronoun used later, 'them,' is plural.
Step-by-step explanation:
The sentence that has an inappropriate shift in number is: If my friend calls, tell them I'm at my violin lesson.
In this sentence, the subject of the sentence, 'my friend,' is singular, but the pronoun used later, 'them,' is plural. To maintain consistency, it would be more appropriate to use the singular pronoun 'him or her' instead of 'them.'