The correct answer is: d) more comfortable
When comparing two items, we use the comparative form of adjectives. In this case, we are comparing the comfort level of two chairs, so we use "more comfortable" to indicate that one chair is superior in comfort to the other. The phrase "much comfortable" is incorrect because "much" is used with non-comparative adjectives to indicate a high degree, while "many more comfortable" is grammatically incorrect. "Very comfortable" is also incorrect in this context because it does not indicate a comparison between the two chairs.