Fusing current is the minimum current at which the fuse element will melt. When a fuse is subjected to excessive current, the fuse element heats up due to the Joule heating effect. This heating causes the fuse element to melt and open the circuit, thereby protecting the electrical equipment and preventing further damage.

Here's a detailed explanation of each option:

a) Maximum current at which the fuse element will get heated: This option is incorrect because the maximum current at which the fuse element will get heated is not necessarily the fusing current. The fuse may get heated at currents below the fusing current, but it will not melt and open the circuit.

b) Rated current of a fuse: This option is incorrect because the rated current of a fuse is the maximum current that the fuse can safely carry continuously without overheating. It is not directly related to the fusing current.

c) The minimum current at which the fuse element will get heated: This option is incorrect because the fuse element can get heated at currents below the fusing current. However, it will not melt and open the circuit unless the current exceeds the fusing current.

d) The minimum current at which the fuse element will melt: This option is correct. The fusing current is the minimum current at which the fuse element will melt and open the circuit. It is the threshold current beyond which the fuse provides protection by interrupting the flow of current.

The fusing current is determined by the physical properties of the fuse element, such as its melting point and cross-sectional area. It is important to select a fuse with a fusing current that is appropriate for the electrical circuit it is protecting. If the fusing current is too high, the fuse may not provide adequate protection and can lead to damage or failure of the equipment. On the other hand, if the fusing current is too low, the fuse may unnecessarily open the circuit under normal operating conditions.

In summary, the fusing current is the minimum current at which the fuse element will melt and open the circuit. It is essential to select a fuse with an appropriate fusing current to ensure proper protection of electrical equipment.