Final answer:
The nurse should prepare for electrical cardioversion when the client's ECG changes to supraventricular tachycardia.
Step-by-step explanation:
When a client's ECG changes to supraventricular tachycardia, the intervention that the nurse should prepare for is c) Electrical cardioversion. Supraventricular tachycardia is a rapid heart rate originating from above the ventricles, and electrical cardioversion is a procedure that uses an electrical shock to restore the heart's normal rhythm. This intervention can be highly effective in treating supraventricular tachycardia and bringing the heart rate back to a normal range.