Don't use negligent.
Tell us exactly what you mean, who you imagine to be saying this sentence, and to whom. Then we'll tell you a good way to say it.
Alternatively, if your aim here is to try to learn how to use the word negligent, then please make that clear to us.
As I asked, please tell us who is saying this sentence, and who is listening. Then we'll be able to answer your question.
A teacher is saying this sentence and one of his colleagues is listening that.
Which of the following preposition should I use after 'negligent'?
1) He is negligent of attending his classes.
2) He is negligent in attending his classes.
Good question. Rather than confuse you with my preferences, check this link: https://lingohelp.me/preposition-after-adjective/negligent-in-of-for-to-about/
A teacher is saying this sentence and one of his colleagues is listening.
Which of the following prepositions should I use after 'negligent'?
1) He is negligent of attending his classes. :cross:
2) He is negligent in attending his classes. :cross:
If I mean that a boy [STRIKE]who[/STRIKE] [STRIKE]is not mindful to[/STRIKE] doesn't concentrate on his studies and doesn't take [STRIKE]it[/STRIKE] them seriously, [strike]then[/strike] what [STRIKE]appropriate[/STRIKE] word should I use?
A teacher is saying this sentence and one of his colleagues [STRIKE]is listening that[/STRIKE] hears it.
I don't think that's very helpful. The preposition used depends on the intended meaning. It isn't a matter of just looking at which one is more frequently used, and it isn't really a matter of preference.
So your answer to the original Barman post is.....?
Not to use negligent at all because it's the wrong word for that sentence.
Or, to answer very directly the question in the thread title: it depends on the context, and what the speaker means.
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