Final answer:
A patient undergoing a paracentesis should be placed in a supine position, which is lying down on their back, face up, facilitating the procedure and comfort.
Step-by-step explanation:
When positioning a patient for a paracentesis, it is important to choose the correct body orientation for both the comfort of the patient and the efficacy of the procedure. The right position would be supine, meaning the patient is lying down on their back, face up. This position allows for easy access to the abdominal area where the paracentesis is performed and gives the healthcare provider better control during the procedure.