Which question should the nurse ask a client with hypertension experiencing elevated blood pressure and cold symptoms?

Asking whether the client is taking any over-the-counter medicines for their cold is crucial in this situation, especially for a client with hypertension. Many over-the-counter cold medications, particularly those containing decongestants like pseudoephedrine, can induce vasoconstriction and increase blood pressure, potentially exacerbating hypertension. Understanding the client's medication usage allows the nurse to assess potential contributing factors to the elevated blood pressure and to provide appropriate recommendations or adjustments in treatment.

In this context, the other questions, while they might provide useful information, do not directly address the immediate concern of how the client's cold symptoms and current medications could be influencing their blood pressure. For instance, babysitting the granddaughter or getting a flu shot does not relate to the pharmacological management of hypertension or the client’s current health status. Similarly, while vitamin C may have some general health benefits, it does not have a recognized impact on blood pressure or interact significantly with common antihypertensive medications. Thus, focusing on over-the-counter medicines is the most relevant and actionable inquiry for managing the client's hypertension in light of their current symptoms.