Silencing of mRNA, also known as RNA interference (RNAi), is a powerful tool in genetic engineering that can be used to produce transgenic plants with enhanced resistance to various pests and diseases. In the given question, the correct answer is option 'B', which refers to nematodes.

Nematodes are microscopic worms that can cause significant damage to plant roots, leading to stunted growth and reduced crop yields. Developing resistance against nematodes is crucial for sustainable agriculture. Silencing of mRNA can be employed to achieve this resistance by targeting specific genes in the nematodes.

Below is a detailed explanation of how silencing of mRNA can be used to produce transgenic plants resistant to nematodes:

1. Introduction to RNA interference (RNAi):
- RNAi is a biological process that regulates gene expression.
- It involves the introduction of small RNA molecules, called small interfering RNAs (siRNAs), into the cells.
- These siRNAs bind to complementary mRNA molecules and prevent their translation into proteins, effectively silencing the gene.

2. Identifying target genes in nematodes:
- To develop resistance against nematodes, specific genes involved in their growth, development, or pathogenicity can be targeted.
- Extensive research is conducted to identify such genes, which can vary depending on the nematode species.

3. Designing siRNAs:
- Once the target genes are identified, siRNAs can be designed to specifically bind to the mRNA transcripts of these genes.
- Careful consideration is given to the siRNA design to ensure high specificity and efficiency.

4. Introduction of siRNAs into plants:
- The designed siRNAs are introduced into plant cells using various methods, including genetic transformation techniques.
- The siRNAs can be delivered as transgenes, which are stably integrated into the plant's genome, or as temporary expressions using viral vectors.

5. Silencing the target genes in nematodes:
- Once the siRNAs are expressed in the transgenic plants, they are transported to the nematodes upon infection.
- The siRNAs then bind to the complementary mRNA molecules of the target genes in the nematodes.
- This binding triggers the degradation of the mRNA molecules, preventing the translation of the target genes into functional proteins.
- Without these proteins, the nematodes are unable to carry out essential functions, leading to their reduced survival and reproduction.

6. Resulting resistance to nematodes:
- As the target genes essential for nematode growth and pathogenicity are silenced, the transgenic plants exhibit enhanced resistance to nematode infestations.
- The damage caused by nematodes to the plant roots is significantly reduced, resulting in improved crop yields.

In conclusion, silencing of mRNA or RNA interference can be used to produce transgenic plants resistant to nematodes. By specifically targeting and silencing genes involved in nematode growth and pathogenicity, the transgenic plants exhibit enhanced resistance to nematode infestations, leading to improved crop productivity.

Option "b" is correct.. nematodes...