Final answer:
Pilocarpine is the drug that could be used to treat the condition of salivary gland hypofunction and associated dry mouth caused by radiotherapy.
Step-by-step explanation:
The drug that could be used to treat the condition of salivary gland hypofunction and associated dry mouth caused by radiotherapy is Pilocarpine. Pilocarpine is a cholinergic agonist that stimulates the parasympathetic nervous system, promoting the production of saliva. By increasing salivary flow, it can help alleviate the symptom of dry mouth. Other options such as Atropine, Diphenhydramine, Ipratropium, and Nicotine are not appropriate choices for treating dry mouth caused by salivary gland hypofunction.