The phrase, 'Do good and forget it' means 'If you have done something right, you should not spend time congratulating yourself. You should rather forget it'. The reason is that one tends to expect something in return for the good that they do. If such a gesture is not reciprocated, one could get disappointed or resentful. Hence, it is better to do the right thing and forget it.
Given the above meaning, the word, 'forget' in option D is the correct answer.
The other words/ phrase given in options A, B and C do not complete the proverb. Hence, these options are incorrect.