The genetically engineered insect-resistant cotton called Bt cotton has been developed by inserting a gene from a/an bacterium. This technology involves the transfer of a specific gene from the bacterium Bacillus thuringiensis, which produces a protein toxic to certain insects, into the cotton plant.

Option 1 states that the gene is inserted from an animal, which is not correct. The correct source of the gene is a bacterium, not an animal.

Option 3 states that the gene is inserted from a fungus, which is incorrect. The gene is not derived from a fungus but from a bacterium.

Option 4 states that the gene is inserted from a plant, which is also incorrect. The gene is not derived from a plant but from a bacterium.

Therefore, the correct answer is option 2, bacterium, as it accurately describes the source of the gene used in genetically engineering Bt cotton.