In a reverse-biased diode, the current is very small and remains almost constant (known as reverse saturation current) until the breakdown voltage is reached. At the breakdown voltage (critical limit), the current increases drastically. Thus, Assertion A is true.
The small current in reverse bias is due to the flow of minority charge carriers, not majority charge carriers. The drastic increase in current at breakdown is due to the generation of a large number of electron-hole pairs (avalanche or Zener breakdown). Thus, Reason R is false.
Answer: A is true but R is false