I received the prompt as an audio to transcribe; I wrote it in the plural form (Ils marchent nus) and it was marked wrong. However, I don't think there would be a detectable difference in the sound; aren't the singular and the plural homophones, here? If anyone can explain why it has to be only singular, I'd appreciate the clarification--thanks.
It should be marked correct. It sounds the same as "Il marche nu". I googled "Ils marchent nus" and got this link to a newspaper article, so some people really do it.
This reminds me of a scene on the TV show "Three's Company" from the 1970's. Their landlord had been playing strip poker and lost all his clothes and was going home, wearing a barrel, when he was stopped by a cop. He had to hold the barrel with both hands and the cop wanted him to do something, which I forget, but it would require him to take his hands off the barrel and he was "nu" under the barrel, so he said, "Have a heart". It was funny.