"Neutral register" is a moving target. "Wordy" can range from slightly informal to neutral, and "verbose" is usually a bit formal, but can be neutral. "Prolix" is rare. Personally, I would choose "wordy" most often.

'verbose' would be my choice for a neutral register. 'wordy' is possible, but it's informal and can be slightly critical (depending on context) 'long-winded' is critical and informal - don't use this ! 'prolix' I've never used.

and by "neutral register"you mean you trying to not be offensive, correct? Which is difficult. I would be able to tell a student that a writing passage is "wordy" if I wanted them to be more concise with their word choices or sentence construction. And I have a colleague, a music teacher, who tends to be long winded at his students' concerts- but I would not tell him that, nor talk about him that way to others. It would hurt his feelings. Verbose is very formal, and useful language if you want to keep the formality, however my students, ages 10-13, don't usually recognize that word, so I don't keep it in my daily vocabulary, though I might use it with another teacher...just so I don't forget it. And honestly I had to look up the the other one.

"Talkative" is beautifully neutral - positive or negative, according to the ear of the beholder. "Chatty" works this way too; but these are both really only for spoken English. For written English, you might try some formulation like "his answers were exhaustive", or "full".

Hi please can you help me to improve my English language