"Bărbaţilor" is the plural, comes from "bărbaţi"
"Barbatului" is the singular, from "bărbat"
So this one translates as "mens' pants".
If you are you referring to a context in which "Pantalonii bărbaților sunt largi" could be translated as "man's pants" (more pants belonging to a single man), it would be wrong.
Also, "mens' pants" (as in pants designed for men) is usually referred in Romanian as "pantaloni bărbăteşti". For example, in a shop "Aveţi pantaloni bărbăteşti?" - "Have you got mens' pants?"
Feel free to ask me if you have questions, I'm a native speaker and I would gladly answer.
As nunes89 writes, he refers to the multiple choice exercise. Here you can choose between "Pantalonii bărbaților/bărbatului sunt largi". You have no translation, the question is just which word gives a correct sentence. And I agree with nunes that both do but only the first is accepted. I am reporting this.