Synthetic auxins are employed for

Options

A. Ripening of fruits
B. Increasing size of fruits
C. Killing weeds
D. Preventing elongation of internodes

Answer

### Step-by-Step Solution:

1. **Understanding Synthetic Auxins**: Synthetic auxins are chemical compounds that mimic the natural plant hormone auxin. They are used in various agricultural practices due to their growth-regulating properties.

2. **Identifying Common Synthetic Auxins**: Some common synthetic auxins include 2,4-D (2,4-Dichlorophenoxyacetic acid) and 2,4,5-T. These compounds are primarily used in herbicides.

3. **Applications of Synthetic Auxins**: Synthetic auxins have several applications, including:
- Promoting root formation in cuttings.
- Regulating fruit development.
- Controlling weed growth.

4. **Analyzing the Options**:
- **Ripening of Fruit**: While auxins can influence fruit development, they are not primarily used for ripening.
- **Increasing Size of Fruit**: Auxins can help in fruit development but are not specifically used for increasing fruit size.
- **Killing Weeds**: Synthetic auxins like 2,4-D are widely recognized for their effectiveness as herbicides, which kill unwanted plants (weeds).
- **Preventing Elongation of Internodes**: Auxins generally promote elongation rather than prevent it.

5. **Conclusion**: Based on the analysis, the primary employment of synthetic auxins is for killing weeds, making the correct answer the third option.

### Final Answer:
Synthetic auxins are employed for killing weeds.