Final answer:
The most important equipment for the nurse to have during carotid sinus massage is a padded tongue blade.
Step-by-step explanation:
The equipment that is most important for the nurse to have ready when performing carotid sinus massage on a client with supraventricular tachycardia is a padded tongue blade. This is because a padded tongue blade is used to apply pressure to the carotid sinus in the neck to help slow down the heart rate. The other options listed, such as an emesis basin, magnesium sulfate, and a resuscitation cart, are not necessary for this procedure.