Option 1: Sunflower - Sunflower oil is commonly used for cooking and is edible. Therefore, sunflower is an edible oilseed.
Option 2: Cottonseed - Cottonseed oil is toxic and contains a substance called gossypol, which can be harmful if consumed in large quantities. Therefore, cottonseed is not edible and is the correct answer to the question.
Option 3: Sesamum (Sesame) - Sesame seeds are commonly used in cooking and are edible. Sesame oil is also commonly used for cooking purposes. Therefore, sesamum is an edible oilseed.
Option 4: Groundnut (Peanut) - Groundnut oil is commonly used for cooking and is edible. Therefore, groundnut is an edible oilseed.
In conclusion, the oilseed that is not edible is option 2, cottonseed.