The correct answer is Preventing electrical fires in buildings.

An Earth Leakage Circuit Breaker (ELCB), also known as a Ground Fault Circuit Interrupter (GFCI), is a safety device primarily designed to protect against electric shock and prevent electrical fires. It works by detecting small imbalances in the current flowing through the live and neutral wires. If there's a leakage current – for instance, due to a fault in an appliance where current is leaking to earth through a person or other conductive material – the ELCB rapidly interrupts the power supply. This immediate interruption minimizes the risk of electric shock and prevents overheating that could lead to a fire.

Let's look at why the other options are incorrect:

• Protecting electrical appliances from overload: This is the function of a circuit breaker or fuse, not an ELCB.
• Protecting high-voltage transmission lines: High-voltage transmission lines utilize different protection systems, typically more sophisticated and robust than ELCBs.
• Detecting voltage fluctuations in the system: Voltage fluctuations are primarily detected and managed by voltage regulators or other voltage-monitoring devices.

Therefore, the main application of an ELCB is its crucial role in fire prevention within buildings by swiftly detecting and cutting off dangerous leakage currents.