Introduction:
In order to protect cotton plants from cotton bollworms, scientists introduced specific genes into cotton that produce proteins toxic to these pests. The most commonly used genes in genetically modified cotton are CrylAc and CryllAb.

Explanation:
CrylAc gene:
- The CrylAc gene comes from Bacillus thuringiensis (Bt), a soil bacterium that produces a toxin harmful to many insect pests.
- This gene codes for a protein called CrylAc, which is toxic to the larvae of cotton bollworms.
- When the cotton plant expresses this gene, it produces the CrylAc protein, which binds to the gut lining of the cotton bollworm larvae, causing the gut to rupture and leading to the death of the pest.

CryllAb gene:
- The CryllAb gene is another variant of the Bt gene, also derived from Bacillus thuringiensis.
- It codes for a slightly different protein called CryllAb, which is also toxic to cotton bollworms.
- Similar to CrylAc, when the cotton plant expresses this gene, it produces the CryllAb protein, which acts in the same way by binding to the gut lining of the cotton bollworm larvae and causing their death.

Why CrylAc and CryllAb?
- Cotton bollworms (Helicoverpa armigera) are among the most destructive pests for cotton crops, causing significant yield losses.
- The use of CrylAc and CryllAb genes in genetically modified cotton has been proven to effectively control these pests.
- These genes provide an environmentally friendly alternative to chemical insecticides, as the proteins produced by the genes are specific to cotton bollworms and do not harm other organisms.

Conclusion:
The correct answer is option 'c' (CrylAc and CryllAb). These genes were introduced in cotton to protect it from cotton bollworms as they produce toxic proteins that specifically target and kill these pests, providing an effective and environmentally friendly pest control method for cotton crops.