The correct answer is 'so far as I have told him.' Key Points - Let us look at the usage of the given options: - So much: So much is used before singular uncountable nouns. - Example: I have never seen so much wealth in my life. - Whichever: used in order to indicate that it does not matter which of the possible alternatives happens or is chosen. - Example: Whichever way you look at it, nuclear power is the energy of the future. - So far: So far means 'up until here' or 'up until now'. - Example: I have only met half of the staff so far. - What: We can use what to ask for information about things and actions. - Example: She asked me what my address was. - Correct sentence: He knows only so far as I have told him.