The answer is: C. Vulgar
Coarse, unrefined, oafish, and blunt are all words that can be used to describe someone who is lacking in social graces or sophistication. However, vulgar is the only word that has a negative connotation of being offensive or indecent.
Coarse can mean “rough” or “unpolished,” as in “coarse language.” Unrefined can mean “not cultured or sophisticated,” as in “unrefined manners.” Oafish can mean “clumsy or awkward,” as in “an oafish lout.” Blunt can mean “direct or outspoken,” as in “a blunt answer.”
Vulgar, on the other hand, can mean “offensive to good taste or manners,” as in “vulgar language.” It can also mean “crude or uncultured,” as in “a vulgar person.”
In the context of the question, the word that is different from the rest is vulgar because it is the only word that has a negative connotation. The other words, coarse, unrefined, oafish, and blunt, can all be used in a negative or positive way, depending on the context.