Which one of the following plants shows vexillary aestivation and diadelphous stamens?

Concept:

• Aestivation is defined as the mode of arrangement of sepals or petals in floral bud with respect to the other members of the same whorl.
• When the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel); this type of aestivation is known as vexillary aestivation.
• Stamens in a flower may either remain free (polyandrous) or may be united in varying degrees. When the stamens  are united into one bunch or one bundle are called monadelphous, and when united into two bundles are called diadelphous.​
• Vexillary aestivation and diadelphous stamens are the characteristic features of the family Fabaceae. 

Explanation: 

Option 1:  Solanum nigrum belongs to Solanaceae.  Therefore, this is the incorrect option.

Option 2:   Colchicum autumnale (colchicine) belongs to the family Liliaceae. Therefore, this is the incorrect option.

Option 3:  Pisum sativum (garden pea) belongs to the family Fabaceae.  Therefore, this is the correct option.

Option 4:  Allium cepa (onion)  belong to the family Liliaceae. Therefore, this is the incorrect option.

So, the correct answer is option 3.