Select the correct option regarding transpiration :-

Options

A. Rate of transpiration `prop (1)/("Root shoot ratio")`
B. Rate of transpiration `prop` Thickness of cuticle
C. Rate of transpiration `prop (1)/("Atmospheric pressure")`
D. Antitranspirants are not useful in dry farming

Answer

To solve the question regarding transpiration, we will analyze each of the provided options step by step and determine which one is correct.

### Step-by-Step Solution:

1. **Understanding Transpiration**:
- Transpiration is the process of water loss from plants, primarily through small openings called stomata. It plays a crucial role in the upward movement of water from roots to leaves via xylem.

2. **Analyzing Option 1**:
- **Statement**: "Rate of transpiration is inversely proportional to root-shoot ratio."
- **Analysis**: A higher root-shoot ratio (more root length compared to shoot length) leads to increased water absorption and thus higher transpiration rates. Conversely, a lower root-shoot ratio results in less water absorption and lower transpiration rates. Therefore, this statement is incorrect; the rate of transpiration is directly proportional to the root-shoot ratio.

3. **Analyzing Option 2**:
- **Statement**: "Rate of transpiration is proportional to thickness of cuticle."
- **Analysis**: The cuticle is a waxy layer on the epidermis that protects the plant. A thicker cuticle reduces water loss, meaning that as the thickness of the cuticle increases, the rate of transpiration decreases. Thus, this statement is also incorrect; the rate of transpiration is inversely proportional to the thickness of the cuticle.

4. **Analyzing Option 3**:
- **Statement**: "Rate of transpiration is inversely proportional to atmospheric pressure."
- **Analysis**: When atmospheric pressure is low, it promotes the diffusion of water vapor from the stomata, leading to increased transpiration. Conversely, high atmospheric pressure reduces diffusion and thus decreases transpiration. This statement is correct; the rate of transpiration is indeed inversely proportional to atmospheric pressure.

5. **Analyzing Option 4**:
- **Statement**: "Anti-transpirants are not useful in dry farming."
- **Analysis**: Anti-transpirants are chemicals that reduce transpiration by closing stomata, which can be beneficial in dry farming conditions to conserve water. Therefore, this statement is incorrect; anti-transpirants are useful in dry farming.

### Conclusion:
After analyzing all the options, the correct answer is **Option 3**: "Rate of transpiration is inversely proportional to atmospheric pressure."