The proverb, 'Grass is always greener on the other side' means : The thing other people have or their situations always look better than your own. But the fact is that the problems faced by those people is not always visible to us. Hence, the word, 'greener' given in option D complete the proverb.
The other words given in options A, B and C do not complete the proverb. Hence, these options are incorrect.