An artificial ecosystem is a crop field.
Explanation:
A crop field is created and maintained by humans. They plant specific crops, manage the soil, and use irrigation to ensure optimal growth, making it a man-made environment.
Why other options are incorrect:
Pond:
Ponds are natural ecosystems. While they might be influenced by human activities like pollution or dam construction, they primarily develop naturally.
Lake:
Lakes are also natural ecosystems. They form over time through geological processes and are sustained by natural factors.
Forest:
Forests are natural ecosystems. They consist of a complex network of trees, plants, and animals that exist without direct human intervention.