what is the name of the plugin where prefix option is presents when creating a thread such as WTS or WTB?
wanted to sell or wanted to buy
Thread prefixes are not a plugin, they are a standard feature. ACP>Configuration>Thread Prefixes
I don't know if there is a specific plugin for this but MyBB 1.6 supports thread prefixes by default.
See AdminCP > Configuration > Settings > Thread Prefixes