The dominant mechanisms for motion of charge carriers in forward and reverse biased silicon P-N junctions are
(a) Drift in forward bias, diffusion in reverse bias
(b) Diffusion in forward bias, drift in reverse bias
(c) Diffusion in both forward and reverse bias
(d) Drift in both forward and reverse bias
(b) In forward biasing the diffusion current increases and drift current remains constant so net current is due to the diffusion.
In reverse biasing diffusion becomes more difficult so net current (very small) is due to the drift.