The correct answer is: D. prevent excessive currents.
A fuse is a safety device that is used to protect electrical circuits from damage caused by excessive current. When a fuse is installed in a circuit, it provides a path for current to flow in the event of an overload. If the current exceeds the rated value of the fuse, the fuse will melt and break the circuit, preventing damage to the appliance or other equipment connected to the circuit.
Option A is incorrect because the main function of a fuse is not to protect the line. The line is protected by the fuse, but this is not the main function of the fuse.
Option B is incorrect because the main function of a fuse is not to open the circuit. The fuse will open the circuit if there is an overload, but this is not the main function of the fuse.
Option C is incorrect because the main function of a fuse is not to protect the appliance. The appliance is protected by the fuse, but this is not the main function of the fuse.
Option E is incorrect because the main function of a fuse is to prevent excessive currents.