Monadelphous androecium is found in:
(a) Compositae
(b) Liliaceae
(c) Malvaceae
(d) Cruciferae

Hint: Monadelphous are the stamens that are united in a single bundle. They are joined together by their filament to form a tube. It is seen in the family of plants which includes hibiscus, okra, cotton, etc. It also includes the cacao plant which is used to produce chocolates and cocoa butter.

Complete answer:
The androecium is the third whorl of the flower which arises inner to the corolla. It is the male reproductive system that is composed of a stamen. A stamen consists of a filament and the anther. The stamens can be freely called as polyandrous or united into a single bundle called monadelphous as in China rose. China rose, also known as Hibiscus, belongs to the family Malvaceae. It is widely cultivated in tropical and sub-tropical regions. Family Malvaceae, also known as Mallows which are used for economic importance including okra, cotton, cacao. The largest genera contain familiar ornamental plants like hibiscus. The flowers are commonly definite or indefinite axillary inflorescence which is often reduced to a single flower but it can also be cauliflorous. They are unisexual or bisexual and are generally actinomorphic. They generally have five valvate sepals, with five imbricate petals. The stamens are five to numerous, but often forming a tube. The ovary is superior with axial placentation. The flowers have nectaries made up of many tightly glandular packed hairs.
So, the answer is, ‘Malvaceae.’

Note: Even if an ornamental flower like hibiscus is aesthetic. Several species of family Malvaceae are pests in agriculture and few are also gardenscapes. Plants like cotton, cacao, okra, and kola nuts are some of the important agricultural crops.