The correct answer is: Option C) Give hints about the characters of Dr. Heidegger's guests.
Withered, melancholy, and foul were words to describe the characters' appareances and personalities, as they were later explicitly revealed to be very old people - and they indeed turn out to be cynical and senile elders who yearn to retrieve their youth -. This description is meant to give the reader a visual idea of how the characters should look like, and predict how they would behave throughout the story based on a brief introduction.