Explanation:
A
The flowers of family Papilionaceae have butterfly shaped corolla, the outermost petal is called standard or vexillum, two lateral petals are similar and clawed, they are called wings or alae and two anterior petals are called a keel this type of petal arrangement is found in the bean, gram, pea, indigofera.
Keel are formed when two anterior small petals are fused at the base. When the keel is formed it forms a concave shape.