Answer:
The sentence that has an inappropriate shift in number is:
If my friend calls, tell them I'm at my violin lesson.
Explanation:
In the sentence "If my friend calls, tell them I'm at my violin lesson," the speaker is at first talking only of one person, which is why he uses "friend" in its singular form. However, later in the sentence, the speaker refers back to the noun "friend" with the pronoun "them", and this is where the inappropriate shift in number happens. "Them" is a plural noun. For the sentence to be correct, the speaker would have to use either "him" or "her" to refer back to "friend".
Answer:
If my friend calls, tell them I'm at my violin lesson.
