Tobacco plants resistant to a nematode have been developed by the introduction of DNA that produced (in the host cells):

Options

A. both sense and anti-sense RNA
B. a particular hormone
C. an antifeedant
D. a toxic protein

Answer

To solve the question regarding the development of tobacco plants resistant to nematodes through the introduction of DNA, we can break down the solution into the following steps:

### Step-by-Step Solution:

1. **Understanding the Problem**:
- The question states that tobacco plants have been genetically modified to resist nematodes, specifically the species *Meloidogyne incognita*, which infects the roots of these plants.

2. **Identifying the Technology Used**:
- The technology employed for developing resistance in these plants is known as RNA interference (RNAi). This technique is used to silence specific genes and can be effective against pests like nematodes.

3. **Mechanism of RNA Interference**:
- RNA interference works by introducing double-stranded RNA (dsRNA) into the host cells. This dsRNA can be derived from both sense and antisense RNA strands. The presence of this dsRNA leads to the degradation of complementary mRNA, thus preventing the expression of genes that would otherwise facilitate nematode infection.

4. **Role of Agrobacterium**:
- To introduce the desired DNA into the tobacco plant cells, a vector such as *Agrobacterium tumefaciens* is often used. This bacterium can transfer DNA into plant cells, allowing for the expression of the RNAi mechanism.

5. **Conclusion**:
- By introducing DNA that produces both sense and antisense RNA, the plants can effectively silence the genes that would allow nematodes to infect them, thereby providing resistance.

### Final Answer:
The correct answer to the question is: **both sense and antisense RNA**.

---