Final answer:
Bethanechol is used to treat urinary retention in MS by stimulating bladder contractions, in contrast to drugs that increase urination by inhibiting salt reabsorption.
Step-by-step explanation:
The drug that can be prescribed to treat urinary retention in patients with Multiple Sclerosis (MS) is bethanechol (option c). Bethanechol is a muscarinic agonist that specifically targets the muscarinic receptors in the bladder, stimulating bladder contractions and improving the ability to urinate. This is contrasted with the side effects of some drugs that increase the frequency of urination by working on the kidneys to inhibit reabsorption of salts, which leads to increased urine production.