Final answer:
The nurse should instruct the client and their partner to take the medication and use a condom during intercourse until cultures are negative.
Step-by-step explanation:
The nurse should provide the client with the instruction B. "You and your partner need to take the medication and use a condom during intercourse until cultures are negative."
Treating both partners simultaneously is important in the case of Trichomonas vaginalis infection because the infection can easily spread between sexual partners. By treating both partners, the chances of reinfection or ongoing transmission are minimized, leading to a more effective treatment plan.