In this question, we are asked to identify the ecosystem that is NOT man-made.

Option 1 - Orchards: Orchards are man-made ecosystems where trees are grown for the purpose of producing fruits. They are created and maintained by humans, so this option is not the correct answer.

Option 2 - Home aquarium: A home aquarium is a man-made ecosystem where aquatic plants and animals are kept in a tank. It is created by humans, so this option is also not the correct answer.

Option 3 - Botanical gardens: Botanical gardens are man-made ecosystems where a wide variety of plants are cultivated for educational, scientific, and aesthetic purposes. Humans design and maintain these gardens, so this option is not the correct answer.

Option 4 - Grassland: Grasslands are natural ecosystems characterized by an abundance of grasses. They are not created or manipulated by humans, so this option is the correct answer.

Therefore, the correct answer is option 4 - Grassland.