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 significantly, Abu Bakar's conversion is historically cited as one of the very first major ones. B is incorrect because Ali (RA)'s conversion happened later than some others mentioned. C is incorrect because Usman (RA) converted at a point in time after the initial wave of early converts.