What is negetive marking in Yahoo answers ?

I believe you're referring to the thumbs up/thumbs down feature. Anybody above Level 2 can give a thumbs up or a thumbs down. In general, they are meaningless; there doesn't have to be a reason to give someone a TU or TD. Some folks will simply TD every answer to a question, or will target some users and give them TDs to make their own answers look better. If enough people TD an answer, it gets hidden from view. But usually that only happens if an answer really IS bad.

You really should not take them personally unless you're answering in a mathematics or other objective category, where a thumbs down could indicate that your answer is wrong. Other than that, I would recommend ignoring them.