B) The second sentence implies that the night watchman is stressed and underfed; the first implies that he is gracefully lean.
Explanation:
While the adjective "slender" makes reference to the man's physique, meaning that he has a healthy weight and the characteristics of a slim person, the adjective "gaunt" refers to the deplorable conditions in which the man is. By being "gaunt", he looks malnourished and is probably in an inadecuate pysichological condition.
