Select the most appropriate option that can substitute the underlined segment in the given sentence. If there is no need to substitute it, select ‘No substitution required’.
Rumour mongering is one of the bad habit.

The grammatical structure "one of the" is always followed by a plural noun because it denotes selection out of many. Therefore, "habit" must be changed to the plural form "habits". Additionally, the positive degree adjective "bad" is contextually appropriate here. Thus, the correct phrase is "one of the bad habits".