Introduction:
In a Bipolar Junction Transistor (BJT), the effective base width is an important parameter that affects the device performance. The effective base width is the distance that minority carriers (holes in an NPN transistor or electrons in a PNP transistor) diffuse into the base region before reaching the collector. It determines the transit time and the current gain of the transistor.

Explanation:
When a reverse voltage is applied across the base-collector junction of a BJT, it creates a depletion region that widens the effective base width. This is known as the reverse biasing of the BJT. The effect of this increase in effective base width is called the Early effect.

Early Effect:
The Early effect is the phenomenon where an increase in reverse voltage across the base-collector junction of a BJT causes a reduction in the effective base width. This effect is due to the widening of the depletion region, which pushes the depletion region closer to the base and reduces the available base region for minority carrier diffusion. As a result, the transit time of the minority carriers through the base region increases, leading to a decrease in current gain and an increase in the base current.

The Early effect is particularly significant in high-frequency applications, where the transit time of carriers becomes a limiting factor. It can cause distortion and reduce the gain of the transistor at high frequencies.

Other options:
- Hall effect: The Hall effect is a phenomenon where a magnetic field applied perpendicular to a current-carrying conductor produces a voltage perpendicular to both the current and the magnetic field. It is not directly related to the reduction in effective base width.
- Zener effect: The Zener effect is the breakdown of a PN junction under reverse bias, leading to a significant increase in current. It is not directly related to the reduction in effective base width.
- Miller effect: The Miller effect is a phenomenon in amplifiers where the input capacitance of a transistor is multiplied by the voltage gain of the amplifier, leading to a decrease in the bandwidth of the amplifier. It is not directly related to the reduction in effective base width.

Therefore, the correct answer is option B) Early effect.