The correct answer is A. Length of heating element should be decreased.
The heating element in an electric kettle is responsible for converting electrical energy into heat energy. The amount of heat energy produced by the heating element is proportional to its power, which is in turn proportional to its resistance. The resistance of a conductor is inversely proportional to its length, so decreasing the length of the heating element will increase its resistance and therefore its power. This will result in the water boiling more quickly.
Option B is incorrect because increasing the length of the heating element will decrease its resistance and therefore its power. This will result in the water boiling more slowly.
Option C is incorrect because the length of the heating element does have an effect on the heating of water. As explained above, decreasing the length of the heating element will increase its resistance and therefore its power, which will result in the water boiling more quickly.
Option D is incorrect because it is not one of the possible answers.