Under which of the following conditions, the absorption of water mainly takes place by active transport?
(i) Low humidity
(ii) Low temperature
(iii) High temperature
(iv) High humidity

Options

A. (i), (ii)
B. (ii), (iv)
C. (i), (iii)
D. Only (iii)

Answer

To determine under which conditions the absorption of water mainly takes place by active transport, we need to analyze each option provided:

1. **Understanding Active Transport**: Active transport is a process that requires energy (ATP) to move substances against their concentration gradient. In the context of water absorption in plants, this process is crucial when conditions are not favorable for passive transport.

2. **Analyzing the Options**:
- **(i) Low Humidity**: In low humidity conditions, plants lose water rapidly through transpiration. To compensate for this loss, they may need to absorb water actively to maintain their turgor pressure and overall health. Thus, active transport is likely to occur here.

- **(ii) Low Temperature**: Low temperatures can slow down metabolic processes, including the rate of transpiration. In such conditions, the need for active transport of water is reduced, as passive transport can suffice. Therefore, active transport is less likely in low temperature conditions.

- **(iii) High Temperature**: High temperatures increase the rate of transpiration, leading to a significant loss of water. To counteract this, plants may rely on active transport to absorb water from the soil, as the demand for water increases. Hence, active transport is likely to occur here as well.

- **(iv) High Humidity**: In high humidity conditions, the rate of transpiration decreases, and plants can absorb water through passive transport without the need for ATP. Therefore, active transport is not primarily needed in high humidity.

3. **Conclusion**: Based on the analysis, the conditions under which the absorption of water mainly takes place by active transport are:
- **Low Humidity (i)**
- **High Temperature (iii)**

Thus, the correct answer is **(i) Low Humidity and (iii) High Temperature**.

### Final Answer:
The absorption of water mainly takes place by active transport under the conditions of **Low Humidity (i)** and **High Temperature (iii)**.