The correct answer is C. specific.
The word “specific” means “relating to or concerning a particular person or thing.” In this case, the scheme proved harmful for the middle income group in particular. This means that the scheme was harmful to the middle income group, but not necessarily to other groups of people.
The word “common” means “occurring, found, or done often; widespread.” In this case, the scheme did not occur or happen often. It was not widespread.
The word “usual” means “customary; habitual.” In this case, the scheme was not customary or habitual. It was not something that happened often or was expected to happen.
The word “general” means “not specific or particular; applying to or affecting all or most people or things.” In this case, the scheme did not apply to or affect all or most people or things. It was specific to the middle income group.