The correct answer is (C) Lakes and gardens.
Lakes are natural ecosystems; gardens are humanmade (artificial) ecosystems.
Source: Chapter 13, Section 13.1 – Eco-system: What are its components?
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The passage explicitly states: "forests, ponds and lakes are natural ecosystems while gardens and crop-fields are humanmade (artificial) ecosystems." Option C is the only pair that matches a natural ecosystem (lake) with an artificial one (garden). Watch out for distractors like Option A (forests are natural, but ponds are also natural — not artificial).