Androecium of Papilionadae family is

The filaments of monadelphous stamens are fused into a single, compound structure. When stamen filaments are fused in two groups, it is termed as diadelphous. The filament of polyadelphous stamens are fused and arranged in more than two groups.
Papilionadeae family is characterized by nine stamen filaments united into a sheath that surrounds the pistil and one separate stamen filament, a condition referred to as diadelphous. 
Therefore, the correct answer is option B.