Keel is characteristic of the flowers of

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Hint: In a flower of the subfamily Faboideae under the angiosperm family Fabaceae, the keel or 2 bottom petals, present below the wings, has a jointed arrangement in order to create a shape resembling a ship's keel.

Complete answer:
There are five styles of petals in the case of pea and bean, of which the most important standard) petal is the one overlapping the 2 lateral petals (wings), which successively overlap the two smallest anterior petals referred to as the keel.
An instance is given by the bean flower. The two lateral petals are known as the wings, whereas, the two lower ones remain normally fused to form a keel like structure enclosing stamens and pistil, are an outsized petal at the highest, called the norm, that grows outside of the others before the flower has opened. The entire architecture is customized for insect pollination or humming birds for a few participants.

Charles Darwin noted that the fertility of papilionaceous flowers depends to a large degree on bee visits, and it is agreed that this arrangement of the corolla formed under the selective pressure of bee pollinators. Linnaeus noted that they contain barely any fragrance, but notable exceptions are the Spanish broom bulbs, sweet pea, scorpion vetch, European yellow lupine, fava bean and numerous species of clover.

Hence, the correct answer is option (A)

Note: Within a boat-shaped bottom petal called the keel petal, which is kept shut by a thin membrane that allows a spring the mechanism, the flowers of Alfalfa keep their reproductive organs tucked away. In order to find nectar and pollen to feed on, cutter bees come up to the flower.