Explanation:

- The sentence discusses the population density in relation to Canada, focusing on how it's spread out across the country.

- Option 1: on - Typically used to describe something physically resting or in contact with a surface, not suitable here.

- Option 2: into - Suggests movement from outside to a point inside, not applicable for describing stationary distribution.

- Option 3: at - Implies a specific point or location, not effective for describing population spread over a large area.

- Option 4: in - Suitable for indicating presence within a larger area, fitting for describing population within Canada.

Therefore, Option 4: in is the correct choice.

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