All floral leaves are placed on the ______ of flower.

Thalamus is the broad base of the on which other organs of the flower rest. The thalamus bears four types of organs - sepals, petals, carpels and the stamens.
Petals are colored accessory floral organs which lie above sepals and they may also provide protection to centrally placed reproductive organs (stamens and carpel).
Sepals are the green outermost leaf like floral organs which protect the flower in bud stage and provide support to outer floral organs.
Ovule is the structure that gives rise to and contains the female reproductive cells.
Thus, the correct answer is option (A), 'Thalamus'.