Rajesh's rustic behaviour astonished the teacher.

Answer: Option C

Rustic : lacking the sophistication of the city; backward and provincial.
Impolite : not having or showing good manners; rude.
Genuine : truly what something is said to be; authentic.
Sophisticated : having, revealing, or involving a great deal of worldly experience and knowledge of fashion and culture.
Awkward : causing difficulty; hard to do or deal with.

Antonym of Rustic is Sophisticated