She . . . . . . . . be very silly at times. A. can B. could C. must D. ought to

The correct answer is can.

• Can is used to express ability or permission. In this case, it is used to express the ability of the woman to be silly.
• Could is used to express possibility or suggestion. In this case, it is used to suggest that the woman might be silly.
• Must is used to express obligation or necessity. In this case, it is used to say that the woman must be silly.
• Ought to is used to express a strong recommendation or expectation. In this case, it is used to recommend that the woman be silly.

In this context, the most appropriate word to use is can. This is because the sentence is about the woman’s ability to be silly, not about her permission to be silly, her possibility of being silly, or her obligation to be silly.