**Meloidogyne incognita** is the nematode that infects the roots of tobacco plants.

**Explanation:**
Nematodes are microscopic, worm-like organisms that can cause damage to plants by feeding on their roots. There are several species of nematodes that can infect tobacco plants, but one of the most common and destructive species is **Meloidogyne incognita**.

Here is an explanation of why **Meloidogyne incognita** is the correct answer:

1. **Meloidogyne incognita** is a species of root-knot nematode that is known to infect a wide range of plant species, including tobacco plants. These nematodes are commonly found in soil and can cause significant damage to the roots of infected plants.

2. The life cycle of **Meloidogyne incognita** begins when the female nematode lays eggs in a gelatinous mass on the surface of the root. The eggs hatch into juvenile nematodes, which then penetrate the root and migrate to the vascular tissue.

3. Once inside the root, the nematodes induce the formation of specialized feeding cells called giant cells. These giant cells provide the nematodes with a nutrient-rich environment, allowing them to feed and reproduce.

4. As the nematodes continue to feed and reproduce, they cause the roots to become swollen and distorted, forming characteristic galls or knots. This can lead to reduced nutrient and water uptake by the plant, resulting in stunted growth, wilting, and ultimately, the death of the plant.

5. The symptoms of root-knot nematode infection in tobacco plants can vary depending on the severity of the infestation. In mild cases, plants may exhibit general decline and reduced yield, while in severe cases, plants may show stunting, yellowing, and wilting.

In conclusion, **Meloidogyne incognita** is the nematode species that infects the roots of tobacco plants. It is a destructive pest that can cause significant damage to the plant's root system and ultimately affect its growth and yield.