Final answer:
A fuse is intended to be the weakest element in a circuit and melts when excessive current flows through it, thus preventing damage to the circuit.
Step-by-step explanation:
A fuse is designed to be the weakest link in a circuit. When excess current is drawn through it, the thin strip of conductive metal inside it melts, opening the circuit and causing a controlled meltdown that will protect the rest of the circuit. The correct answer to the student's question is 'b. Fuse'.
A fuse has a metal strip with a low melting point that, when overheated by an excessive current, permanently breaks the connection of a circuit to a voltage source. This protective action is due to the physical properties of the fuse wire, which is under tension and melts when it exceeds the rated current, thereby safeguarding the circuit components.