Which part of tobacco plant is infected by Meloidegyne incognitia?

The correct answer is Option (3) → Root

Meloidegyne incognitia is a nematode (round worm) that infects the roots of tobacco plants and causes a great reduction in yield. Scientists made tobacco plants resistant to it by RNA interference (RNAi): the plant makes double stranded RNA that silences a specific mRNA of the parasite, so the nematode cannot survive in the transgenic host.

NCERT Extract:
Biotechnological Applications in Agriculture – “… Pest Resistant Plants: Several nematodes parasitise a wide variety of plants and animals including human beings. A nematode Meloidegyne incognitia infects the roots of tobacco plants and causes a great reduction in yield. A novel strategy was adopted to prevent this infestation which was based on the process of RNA interference (RNAi).”