Some indefinite pronouns take a singular verb. Everybody is one of them. In your example it should therefore be 'has stated', not 'have stated'.

There are two separate issues here: 1. The word 'everybody' : this is a singular noun. We say 'Everybody is...' and 'Everybody knows..' and 'Everybody has'. This is why your answer is wrong : it should be 'Everybody HAS stated..', not 'have'. Of course, if a past simple is used, this mistake doesn't arise - if you say 'Everybody stated', this is fine, too. 2. The pronoun used with impersonal or non-gender-specific nouns. There are several options here: Your option, 'his/her', is fine. You could also use 'his or her'. The very outdated masculine-only form still exists ( just 'his'), but it is very rare nowadays. The best option in modern English is the singular 'their'.

I know That I'm wrong but why?Here is the correct sentence: ''Everybody in the office stated their opinion''