Detailed Explanation:

Insectivorous plants (e.g., Venus flytrap, pitcher plant, sundew) grow in nitrogen-deficient soils like bogs, marshes, and wetlands. They compensate by trapping and digesting insects to obtain nitrogen and other essential nutrients.

Water, magnesium, and calcium deficiencies do not trigger insectivory; these nutrients are obtained through roots in normal soils.

Therefore, insectivorous plants grow in nitrogen-deficient soil, making Option 3 the correct answer.