1. Working principle of a photodiode

A photodiode detects light because incident photons generate electron-hole pairs in the depletion region.

• These generated carriers contribute to current.
• In a photodiode, the current due to light is mainly because of minority carriers.

2. Why reverse bias is preferred

When the diode is reverse biased:

• The depletion region becomes wider.
• The electric field across the junction becomes stronger.
• The photo-generated electron-hole pairs are quickly swept across the junction.
• Hence, even a small change in incident light produces a noticeable change in reverse current.

Also, in reverse bias, the current in a normal diode is already very small and is due mainly to minority carriers. Therefore, any fractional change in minority carriers caused by light leads to a significant change in reverse current.

3. Check each option

• A: fractional change in majority carriers produce higher forward bias current
  This is not the reason for photodiode operation. Forward current is dominated by majority carrier injection, not preferred for light detection.

• B: fractional change in majority carriers produce higher reverse bias current
  Reverse current is not mainly due to majority carriers. So this is incorrect.

• C: fractional change in minority carriers produce higher forward bias current
  Photodiodes are not preferably operated in forward bias for detection. Incorrect.

• D: fractional change in minority carriers produce higher reverse bias current
  This matches the operating principle of a photodiode. Correct.

4. Final answer

Therefore, the correct option is:

$\boxed{\text{D}}$