Final answer:
Quinidine is the antiarrhythmic drug most likely responsible for the symptoms of nausea, vomiting, abdominal pain, diarrhea, and tinnitus reported by the client.
Step-by-step explanation:
A client on antiarrhythmic drug therapy complains of nausea, vomiting, abdominal pain, diarrhea, and a ringing sensation in the ears. Among the options provided, Quinidine is the drug most likely to cause these symptoms. Notably, quinidine, an antiarrhythmic medication, has side effects that include tinnitus (ringing in the ears), hearing impairment, vertigo, vomiting, and abdominal pain. Therefore, when assessing a client with these adverse effects, quinidine should be considered as the possible cause of the symptoms.