If you picked D, your vocabulary is getting better. Since "ameliorate" implies making a bad situation or condition (such as tension among friends) more tolerable, Sara's apology in D fits the bill perfectly. Another word that would fit in this sentence is "meliorate." Though not as common, "meliorate" is not only a synonym of "ameliorate," but is also its parent word. "Ameliorate" was coined in the mid-18th century as an alteration of "meliorate," a word from the 16th century that also means "to make or grow better." "Meliorate" traces back to the Latin word "melior," meaning "better."