When the liquid between two ends of a circuit allows electric current to pass through, the bulb of a tester glows. Let's understand this phenomenon in detail.

Explanation:
- The bulb of a tester is a small device that is used to test the presence of an electric current in a circuit. It consists of a filament that emits light when an electric current passes through it.
- When a circuit is complete, i.e., there is a continuous flow of electric current, the tester bulb will glow.
- In the given scenario, the liquid between two ends of the circuit allows the electric current to pass through. This means that the circuit is complete, and there is a flow of electric current from one end to the other.
- As the electric current flows through the circuit, it passes through the filament of the tester bulb. The filament gets heated up due to the resistance offered by the liquid.
- When the filament gets heated up, it starts emitting light, causing the bulb of the tester to glow.
- The glow of the bulb indicates that there is a flow of electric current in the circuit, and the liquid is conducting electricity.
- The color of the tester bulb doesn't change in this scenario. The color of the light emitted by the bulb depends on the material used for the filament, not on the conductivity of the liquid.
- Therefore, the correct answer is option 'C' - the bulb of the tester glows.

In summary, when the liquid between two ends of a circuit allows electric current to pass through, the bulb of a tester glows. This indicates the presence of a complete circuit and the flow of electric current. The glow of the bulb is a result of the filament heating up due to the resistance offered by the liquid, and it does not change the color of the bulb.