The correct answer is option 4. The filament of an electric bulb is made of tungsten because it has a high melting point.

Option 1 - Tungsten is not necessarily cheap. The choice of tungsten for the filament of an electric bulb is not primarily based on cost, but on its physical properties.

Option 2 - While tungsten is indeed durable, this is not the main reason why it is used for the filament of an electric bulb.

Option 3 - Tungsten does have a high light-emitting power, which means it can produce bright light. However, this is not the main reason why it is chosen for the filament material.

Option 4 - The high melting point of tungsten is the key reason why it is used for the filament of an electric bulb. When an electric current flows through the filament, it gets heated up to a very high temperature. Tungsten`s high melting point allows it to withstand these extreme temperatures without melting or breaking. This makes tungsten the ideal material for the filament, as it can glow brightly and emit light for extended periods without getting damaged.