The correct answer is in. The phrase "consist in" is used to specify what something's essential nature, fundamental characteristic, or core essence is. When we say "Success consists in honesty," we are indicating that honesty is the primary and defining quality or principle that constitutes success. It suggests that the very essence or heart of success lies within honesty.
Option A, off, is incorrect. "Consist off" is not a standard or grammatically recognized phrasal verb in English. The preposition "off" typically indicates separation, removal, or a starting point, none of which fit the context of describing the intrinsic quality of success.
Option B, of, is incorrect. While "consist of" is a grammatically correct phrase, it is used to list the individual parts or components that make up a whole, as in "The team consists of five players." However, success isn't typically seen as being made up of separate "parts" of honesty; rather, honesty is viewed as its inherent quality. "Consist in" more accurately conveys this deeper, defining characteristic.
Option D, on, is incorrect. "Consist on" is not a recognized or grammatically correct phrasal verb. The preposition "on" generally indicates position, dependence, or a topic, none of which accurately convey the idea that honesty is the essential nature or defining characteristic of success.