Step 1: Analyze the sentence structure and context.
The sentence requires a subject that is an activity which can be described as "good for health." The options provided are gerunds (verb + -ing acting as a noun). Step 2: Evaluate each option in the context of "good for health."
(1) Walking: Walking is a form of physical exercise and is widely known to have numerous health benefits, such as improving cardiovascular health, strengthening muscles, and helping with weight management. Therefore, "Walking is good for health" makes logical sense.
(2) Writing: While writing can be mentally beneficial and a form of expression, it is not typically categorized as an activity that is universally "good for health" in the same physical sense as exercise.
(3) Waiting: Waiting is a passive state and does not inherently contribute to physical or mental health benefits.
(4) Reading: Reading is an excellent mental activity that can improve cognitive function and reduce stress, but it's not typically described as "good for health" in the broad, common understanding that implies physical well-being. Step 3: Choose the option that best fits the blank.
Among the given choices, "Walking" is the most appropriate word that describes an activity universally considered "good for health." $ $