Seriously I won't expect such an unelaborated question from an experienced Edaboard member who frequently posted in the power electronics section.
I believe you can find many answers yourself, most of it referring back to an exact specification of circuit conditions.
Circuit a) is commonly used, as CataM mentioned, there are better circuits in term of voltage drop. Does it matter in your application? E.g., if the diode is followed by a linear regulator, it usually doesn't matter which part drops the voltage difference. Different with an efficient SMPS.
Circuit b) is only acceptable if the input source is current limited or fused, and the diode rated for the short circuit current.