Explanation

D is correct because Hazrat Abu Bakar (RA) was among the earliest and most emphatic converts. A is incorrect because while Umar (RA) converted emphatically, Abu Bakar's conversion is historically cited as one of the very first major ones. C is incorrect because Usman (RA) converted later than the others listed. B is incorrect because Ali (RA)'s conversion happened after some of the earliest prominent conversions.