The fuse rating must be higher than the normal operating current of the electrical appliance/component. Common fuses are 1A, 3A, 5A, 8A, 10A and 13A. Some teachers make students memorise this (don't know why). Generally speaking, its best to use a fuse rating of 'normal operating current' + 1, and then round up. Factually, all 3 pin plugs used in a home have a 13A fuse installed, and can also be used for this question, but teachers would want you to use a fuse rated slightly higher than the operating current.