Concept:The green colour of the solution is caused by algae, which grow rapidly when the culture vessel is exposed to light.
Explanation:Algae enter the nutrient solution and multiply quickly in the presence of sunlight.
These algae turn the solution green and compete with the plant for oxygen and nutrients, eventually causing the plant to die.
To prevent this, the culture vessel must be kept in darkness so that algae cannot photosynthesise and grow.
Covering the vessel with opaque paper blocks light and stops algal growth, keeping the solution clear.
Thus, the most suitable precaution is to cover the culture vessel with an opaque material.
Answer:C. cover the culture vessel with opaque paper
