The correct answer is option 1: In household insecticides. Allethrin is a synthetic insecticide that is widely used in household insecticides. It belongs to a class of chemicals called pyrethroids, which are derived from the natural insecticide pyrethrum found in chrysanthemum flowers.
Option 2, in glass manufacture, is incorrect. Allethrin is not used in the manufacturing of glass.
Option 3, in steel manufacture, is incorrect. Allethrin is not used in the manufacturing of steel.
Option 4, in paints and varnishes, is incorrect. Although pyrethroids may be used in some paints and varnishes as insect repellents, allethrin specifically is primarily used in household insecticides rather than paints and varnishes.
Therefore, the correct answer is option 1: In household insecticides. Allethrin is commonly used to control and kill insects in and around the home.