"남자의 손톱이 검어요."
Translation:The man's fingernails are black.
I was marked incorrect for using the singular form. However, I believe both singular and plural are correct here. The -들 plural ending is optional, from what I have heard.
It is both or not based on context. If you want to be very specific you can add a -들 to the end of a noun and that makes it plural. That said if it marked you wrong for singular then please use the flag option to let the team know that there is a possible sentence missing.
You usually would use context clues or it would be irrelevent. If you wanted to be 100% certain someone listening knew something is plural, you would add +들 to the end of the word, but before the subject particle. Example: 남자들은 있다.