What adjective would best describe someone who is courteous and sociable?

The adjective "affable" is a fitting choice for someone who is courteous and sociable because it specifically denotes a friendly and approachable demeanor. An affable person is easy to talk to, exudes warmth, and is generally pleasant in social interactions, which perfectly encapsulates the qualities of being courteous and sociable.

In contrast, "aghast" describes a state of shock or horror, which does not relate to sociability or courtesy. "Alleged" pertains to something claimed or stated without proof, typically used in legal contexts, and has no bearing on personality traits. "Adulterated" refers to something that has been made impure or mixed with inferior substances, which is unrelated to the social and polite characteristics being described. Each of these words falls far from the qualities associated with being cordial and friendly, reinforcing why "affable" is the most suitable choice in this context.