Root pressure is the positive pressure that develops in the roots of the plants by active absorption of nutrients from the soil. The active absorption depends on the active accumulation of solute in xylem sap. Root pressure in maximum when transpiration is very low and absorption is high because transpiration is the output of water from a plant, and absorption is the input of water into a plant. If the output is low and input is high, the pressure will be at its greatest.
So, the correct answer is 'Transpiration is very low and absorption is high'.

Explanation:

Root pressure is the pressure exerted by the cells of the root cortex on the water molecules inside the xylem sap of roots. It is responsible for the movement of water from the roots to the stems and leaves. The magnitude of root pressure depends on various factors such as transpiration rate, absorption rate, and osmotic pressure.

Effect of transpiration on root pressure:
Transpiration is the process of water loss from the leaves of plants in the form of water vapor. It creates a negative pressure or tension in the xylem vessels, which pulls water from the roots to the leaves. Thus, high transpiration rate decreases the root pressure.

Effect of absorption on root pressure:
Absorption is the process of uptake of water and minerals from the soil by the roots. It creates a positive pressure or tension in the root cells, which pushes water into the xylem vessels. Thus, high absorption rate increases the root pressure.

Answer:

The correct answer is option 'B' - Transpiration is very low and absorption is high.

When transpiration is very low, the negative pressure or tension in the xylem vessels is reduced. This reduces the pull on the water in the roots, which increases the root pressure. When absorption is high, the positive pressure or tension in the root cells is increased. This increases the push on the water into the xylem vessels, which further increases the root pressure. Hence, root pressure is maximum when transpiration is very low and absorption is high.

Conclusion:

Root pressure is an important factor in the movement of water and minerals in plants. It is influenced by various factors such as transpiration rate, absorption rate, and osmotic pressure. When transpiration is very low and absorption is high, root pressure is maximum.