The correct answer is option 3, Cotton. A BT seed refers to a genetically modified seed that has been developed to contain the Bacillus thuringiensis (BT) gene. This gene helps in producing a protein that is toxic to certain pests, particularly the bollworm, which is a major problem in cotton plants. By using BT seeds, farmers can control the bollworm population without having to rely on chemical pesticides. These BT seeds have been developed specifically for cotton plants, making option 3 the correct answer.

Options 1, 2, and 4 are incorrect because although genetically modified varieties of wheat, maize, and pulses (legume crops) do exist, BT seeds have not been developed for these crops.