Introduction:
A bulb is an electrical device that produces light when an electric current passes through it. It consists of several components that work together to generate light. Among these components, the filament plays a crucial role in producing light.

Explanation:
Glass:
The glass envelope of a bulb serves as a protective covering. It helps to insulate the filament and other internal components from external factors such as dust, moisture, and damage. However, the glass itself does not produce light.

Current:
Electric current is necessary for a bulb to function. When a current flows through the bulb, it energizes the filament, causing it to heat up and emit light. Without an electric current, the bulb will not produce any light. However, the presence of an electric current alone does not guarantee the production of light in a bulb.

Filament:
The filament is the key component of a bulb responsible for producing light. It is typically made of a thin wire, often composed of tungsten. When an electric current passes through the filament, it becomes extremely hot due to the high resistance of the wire. This intense heat causes the filament to glow and emit visible light. The filament's material and design are carefully chosen to withstand the high temperatures generated and provide a consistent light output.

Light:
The main purpose of a bulb is to produce light, which is the desired output. When the filament is heated to a high temperature, it emits visible light. The light emitted may vary in intensity, color, and direction depending on the design and specifications of the bulb.

Conclusion:
While a bulb consists of various components like glass and requires an electric current to function, the filament is the essential component responsible for producing light. Without the presence of a filament, a bulb would not be able to emit light. Therefore, the correct answer to the question is option 'C' - Filament.