Family Fabaceae is subdivided into 3 sub-families as Papilionoideae, Caesalpinoideae and Mimosoideae. These 3 sub-families are easily identifiable by their flowers.  
Papilionoideae flower shows zygomorphic flower (of which symmetry remains intact when it is cut into two equal halves only through one particular vertical plane passing though the center) with 5 petals with 2 banner petal, 2 wing petals and 2 petals which partially fuse together to form boat shaped keel. It possess 10 distinct or fused or diadelphous stamens (stamens with united filaments). 
Caesalpinoideae flower shows slightly zygomorphic flower with 5 petals which are not differentiated into banner, wing and keel. 10 or less stamens which are visible externally. 
Mimosoideae flower are actinomorphic (A flower when cut through any vertical plane passing though the axis then two equal halves are formed and its symmetry remains undisturbed. Such flower is called as an actinomorphic flower). Flowers are small and are crowded together to form pom-pom like structure. Stamens are 10 to many and extend beyond corolla.
Therefore, the correct option is A.