Which of the following is a sign of a stroke?

A sudden headache is indeed a significant sign of a stroke and can often be described as a severe headache that comes on suddenly, sometimes referred to as a "thunderclap headache." This type of headache can indicate a type of stroke known as a hemorrhagic stroke, which occurs when a blood vessel in the brain bursts, leading to bleeding in or around the brain. Recognizing this symptom promptly is crucial, as immediate medical attention can greatly affect outcomes for individuals experiencing a stroke.

Other options, such as frequent coughing or increased appetite, do not typically relate to stroke symptoms. While difficulty walking can indeed be a sign of a stroke, as it may indicate coordination or balance issues caused by brain dysfunction, the specific and sudden nature of a headache makes it a particularly alarming symptom that warrants immediate action.