In an electric kettle water boils in 10 minutes. If it is required to boil the water in 15 minutes, using same supply mains, which of the following is correct ?

length of heating element should be increased

heat, H = VI t = I²tR. Since mains supply is same, so V will be constant. Power rating is also constant. Increasing the length of the heating element will increase the resistance and hence, the water will boil slower.