It is quite unseemly that person should so behave with his elders.

Answer: Option D

Childish : of, like, or appropriate to a child.
Ugly : unpleasant or repulsive, especially in appearance.
Discourteous : showing rudeness and a lack of consideration for other people.
Improper : not in accordance with accepted standards, especially of morality or honesty.
Unseemly : (of behaviour or actions) not proper or appropriate.

Synonym of Unseemly is Improper