To encourage yourself to ask questions rather than simply collect information, you should ask "How does a country's population size contribute to its levels of air pollution?" (Option A).
With this type of question, you are going to find information related to the topic that will make you come up with new questions such as why is the population a factor in air pollution or whether the country's policies prevent air pollution or not.
With the other questions, you will only get information about certain countries like the US or Canada or pollution in relation to the size of the country instead of its inhabitants and policies.