Explanation:

- The context of the passage is about the beneficial qualities of muskmelon during summer, specifically highlighting its water content.

- Option 1: lengthy does not fit because the muskmelon is not characterized by length or duration in this context.

- Option 2: poor is incorrect as it contradicts the positive aspects being discussed about muskmelon's beneficial properties.

- Option 3: wide is irrelevant, as it refers to breadth or scope, which does not apply to water content.

- Option 4: rich is the most appropriate choice. It accurately describes muskmelon as being abundant in water, which is beneficial for hydration.

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