The correct answer is C. 3 and 4 only.
A safety fuse is a device that is used to protect electrical circuits from damage caused by excessive current. It does this by melting when the current exceeds a certain level, which interrupts the flow of electricity.
The working of a safety fuse depends on the heating effect of the current. When current flows through a conductor, it causes the conductor to heat up. The amount of heat generated is proportional to the square of the current. This means that a small increase in current can cause a large increase in heat.
If the current through a fuse exceeds the rated value, the fuse will melt. This will interrupt the flow of electricity and protect the circuit from damage.
The magnetic effect of the current and the chemical effect of the current do not play a role in the working of a safety fuse.