The correct option is C Approximately
Option D is correct because 'in round numbers' means 'an approximate number'. Thus, option D is the correct meaning of the given idiom.
Option A is incorrect because 'roughly' means 'something not precise' whereas 'in round numbers' means 'an approximate number'. Thus, option A is not the correct meaning of the given idiom.
Option B is incorrect because 'strictly' means 'rigidly enforced ' whereas 'in round numbers' means 'an approximate number'. Thus, option B is not the correct meaning of the given idiom.
Option C is incorrect because 'maximum' means 'in large number' whereas 'in round numbers' means 'an approximate number'. Thus, option C is not the correct meaning of the given idiom.