Which among the following parasite infects the roots of tobacco plants and causes a great reduction in yield ?

Meloidegyne incognitia

Among the options provided, the parasite that infects the roots of tobacco plants and causes a significant reduction in yield is "Meloidogyne incognita."

Meloidogyne incognita is a species of plant-parasitic nematode, commonly known as root-knot nematode. These microscopic roundworms infect the roots of various plants, including tobacco plants, by forming galls or knots on the root system. The feeding activity of these nematodes leads to root damage, impairing the plant's ability to uptake water and nutrients, ultimately resulting in stunted growth, reduced yield, and sometimes the death of the plant.

As for the other options:

• Wuchereria is a parasitic worm that causes lymphatic filariasis in humans, commonly known as elephantiasis, and does not infect tobacco plants.
• Bacillus thuringiensis (Bt) is a bacterium that produces proteins toxic to certain insects and is used in some genetically modified crops to confer resistance against specific pests. It is not a parasite that infects tobacco plants but rather a biopesticide used to control insect pests.

Therefore, the correct answer related to the parasite that infects tobacco plants' roots and reduces yield is "Meloidogyne incognita" or root-knot nematode.