Concept:The underlined word accidentally means happening by chance or without intention.Explanation:The sentence says the writer can vouch that the glass was broken accidentally and not in a planned or intentional way.The word that is most nearly opposite to “accidentally” is “deliberately.”“Deliberately” means consciously and intentionally, or done on purpose.This fits the gap correctly because the sentence contrasts an accidental act with an intentional one.Answer:D. deliberately