Answer:
Option B, In a dark closet
Step-by-step explanation:
The student wants to see if the plants need sunlight to grow or not.
They kept two plants in sunlight.
Hence, they must put the remaining plants in dark so that they can compare the difference in growth of two set of plants.
In case if the growth is plants kept in sunlight is higher than the growth in plants kept in dark (keeping all other variables constant), then it will be clear or proved that sunlight is essential for growth of plants
Hence, option B is the correct answer