The correct option is B On
Option B is the correct answer as the English phrase ‘on average’ is used to indicate what is usually true. Hence, Option B brings out the correct meaning of the sentence. Preposition 'in' is used to show a state or condition, 'out of' is generally used to show what something is made from, and 'of' is used to indicate reference or amount generally. Options A, C and D do not bring out the intended meaning of the sentence and hence, cannot be used here.