The twins' heredity and upbringing were identical in nearly every respect, yet one child remained unfailingly sanguine even in times of stress while her sister was prone to angry outbursts that indicated an exceptionally choleric ____.

(E) temperament and (F) humor
The main clues suggest that one twin is described as sanguine and the other as choleric, with phrases like “even in times of stress” and “angry outbursts” hinting at words used to describe personality. “Temperament” is a suitable synonym for personality. Although “humor” is often used to refer to comedy, it can also denote personality, especially when paired with words like sanguine and choleric, which stem from the ancient belief that temperament was influenced by levels of different bodily fluids or humors. The remaining choices—environment, disposition, and constitution—do not align with the clues provided. “Environment” pertains to one’s surroundings, while the other two words are related to the physical rather than the mental aspect. Therefore, “temperament” is the best fit in describing the twins’ personalities.