The dominant mechanisms for motion of charge carriers in forward and reversed biased silicon p-n junctions are :
Diffusion in forward bias, drift in reverse bias
Diffusion in both forward and reverse bias
Drift in both forward and reverse bias
None of these
The correct option is A Diffusion in forward bias, drift in reverse bias
$$\Rightarrow $$ In forward bias we apply voltage in a direction opposite to that of barrier potential p-side to positive terminal, n-side to negative terminal of battery. So electrons in the n-side, holes in p side pushed towards the junction. Results in increased diffusion.
$$\Rightarrow $$ In reverse bias, electrons in n-side, holes in p-side pushed away from function.
n-side to positive terminal p-side to negative terminal.