Correct Answer: B. Corn Borer

Explanation:
The given question is related to the control of pests in agriculture. The pests mentioned in the options are different types of insects that commonly attack crops. Among the given options, the correct answer is option B, which is the Corn Borer.

1. Stem Borer:
- Stem borers are a type of insect pest that attack the stems of plants, including crops like rice, maize, sugarcane, sorghum, and millets.
- They bore into the stems, causing damage to the vascular tissues, which affects the nutrient and water flow within the plant.
- This leads to stunted growth, wilting, and ultimately, reduced crop yield.

2. Corn Borer:
- Corn borer, also known as European corn borer or maize borer, is a common pest that affects maize or corn crops.
- The larvae of these insects feed on the leaves, stems, and developing ears of the corn plants.
- This feeding activity weakens the plant, reduces photosynthesis, and causes yield losses.
- The damage caused by corn borers also makes the plants more susceptible to diseases.

3. Cotton Bollworm:
- Cotton bollworm, also known as Helicoverpa armigera, is a major pest of cotton crops.
- The larvae of this insect feed on the reproductive parts of the cotton plant, including the flower buds, flowers, and bolls (seed pods).
- This feeding damage results in reduced cotton yield and quality.
- Cotton bollworm is also known to attack other crops, including corn, soybean, tomato, and tobacco.

4. Root Pest:
- The term "root pest" is a general category that includes various pests that attack the roots of plants.
- These pests can be nematodes, insects, or even fungal pathogens.
- Root pests can cause severe damage to crops by feeding on or damaging the root system.
- This affects the plant's ability to uptake water and nutrients, resulting in stunted growth, wilting, and reduced yield.

Therefore, among the given options, the correct answer is option B, which is the Corn Borer.