Option 1 -> frivolity means lack of seriousness, which is a synonym of levity, not an antonym.
Option 2 -> witticism refers to a clever or humorous remark, which relates to humor but is not the opposite of levity.
Option 3 -> criticism means expressing disapproval or judgment, which is unrelated to the concept of seriousness vs. lightheartedness.
Option 4 -> gravity means seriousness or solemnity, which is the direct opposite of levity (lightheartedness or lack of seriousness).
Hence, Option 4: gravity -> Levity means inappropriate lightheartedness or lack of seriousness in a serious situation. The antonym is gravity, which means seriousness and solemnity of manner. In the context, her levity (lightheartedness) was appalling given the terrible situation, and gravity would have been the appropriate demeanor -> correct