Final answer:
Cindy needs to collect data on the reading comprehension level of her parents to determine which newspaper is easier for them to understand.
Step-by-step explanation:
Cindy wants to determine which of the newspapers her parents read is easier to understand. To make this determination, Cindy needs to evaluate the readability of these newspapers. The relevant data that Cindy requires would be D. The reading comprehension level of her parents. This would allow her to understand which newspaper aligns best with the reading abilities of her mom or dad, and hence, which one is easier for them to understand. The birthdates, the cost of the newspapers, or the educational levels of her parents are less directly relevant to determining the readability of the newspapers.