Option 1: Keeping the plants in the shade may not necessarily increase the rate of absorption of water through roots. While shade can help in preventing water loss through evaporation, it does not directly affect the absorption process.

Option 2: Keeping the plants in dim light also does not directly increase the rate of water absorption. Light plays a role in photosynthesis, but it is not directly related to the uptake of water.

Option 3: Keeping the plants under a fan can increase the rate of water absorption through the roots. The movement of air created by the fan helps in transpiration, which in turn increases the rate of water movement from the roots to the leaves.

Option 4: Covering the plants with a polythene bag can actually decrease the rate of water absorption. It creates a barrier that restricts the movement of air and can lead to excess moisture accumulation around the roots.

Therefore, the correct answer is option 3 - keeping the plants under a fan.