Final answer:
The most common adverse effect to monitor for in an adult client taking hydroxyzine for nausea is dry mouth. This side effect is due to the anticholinergic properties of the drug. Therefore, the correct option is b.
Step-by-step explanation:
An adult client being administered hydroxyzine for nausea should be monitored for several potential side effects. However, among the options provided, dry mouth is a common adverse effect of hydroxyzine. Hydroxyzine is an antihistamine that has anticholinergic properties, which can lead to drying of the oral mucosa. Although hydroxyzine can cause a range of side effects, including agitation, anxiety, sleep problems, nausea and vomiting, abnormal heart rhythms, low blood pressure, and muscular problems, dry mouth is more commonly associated with its anticholinergic activity and hence the correct answer is (b) dry mouth.
Other options such as palpitations, thrombocytopenia, and hypertonic muscle tone might be related to different types of medications or conditions, but they are not typically prominent side effects of hydroxyzine when used to treat nausea. For example, palpitations could indeed be related to heart rhythm issues, but they are less commonly reported with hydroxyzine usage than dry mouth.