"He eats the salad."
Translation:Lui mangia l'insalata.
Maybe not preferred, but yes, you can leave out the pronoun if there will be no confusion as to whom you're referring. BTW, questions can appear in different formats with the same discussion appearing for the same correct answer, i.e. some students on this forum might have had a multiple choice question, others a translation, and still others a "type what you hear" exercise.
Mine had no option for "lui"! Only for mangia l' insalata. I thought therefore, that the lesson was that you don't have to signify "lui". If you're talking about a male, you don't have to define who mangia anything. Is that the point of my lesson? If not, why wasn't Lui presented as an option?
"Mangia l'insalata" could mean either "He eats the salad" ("Lui mangia l'insalata"), Or "She eats the salad" ("Lei mangia l'insalata"), As the verb is conjugated for third-person singular, The pronoun can be dropped, And would often be inferred from context. If it's not clear who you are talking about, You could add "Lei" or "Lui" to specify.
There's nothing we can do about a correct answer not being accepted, Especially if you don't even share the full answer. If your answer was not accepted, But you feel it should be, I'd recommend double checking to make sure there are no errors, And then once you're sure it's correct, Reporting it as "My answer should be accepted".