Trimerous flower occur in-
A. Monocots
B. Dicots
C. Gymnosperms
D. All the above

Hint: A flowering grass-like plant (angiosperms) or grass family group whose flowers are short-lived and whose perianth is not divided into calyx and corolla. Around two-thirds of them are mainly insects (zoophilous).

Complete answer:
Option A is correct. The perigone is composed of two alternating trimerous whorls of tepals in almost all cases, being homochlamydeous, without distinction between calyx and corolla. Both whorls are corollin (petal-like) in zoophilous (pollinated by animals) taxa. The time of flower opening (Anthesis) is typically fleeting (short lived). Thermonastic opening and closing are seen (responsive to changes in temperature) by some of the more persistent perigones.
Option B is incorrect. Typically, dicots have four or five sepals and four and five petals. There are typically four, five, ten or multiple stamens in dicots. Count the number of sepals, stamens, and petals. Dicots are the most common garden plants, shrubs and trees, and wide-leafed flowering plants like magnolias, roses, geraniums, and hollyhocks.
Option C is incorrect. Angiosperms, also referred to as flowering plants, have seeds enclosed inside an ovary (usually a fruit), while gymnosperms have no flowers or fruits, and have seeds on the surface of scales or leaves that are unenclosed or "naked." Seeds from gymnosperm are also configured as cones.
Option D is incorrect. The sections of the flower in monocots are normally in threes. Just one stigma typically remains, with three stamens and three petals and three sepals. The number of petals in dicot flowers is 4 or 5 or a multiple of 4 or 5, while gymnosperms have no flowers or fruit on the surface of scales or leaves, and have unclosed or 'naked' seeds.

So, the correct answer is Option A.

Note: Seeds with a single cotyledon, parallel-veined leaves, distributed vascular bundles in the stem, the absence of a typical cambium, and an adventitious root system are distinguished by monocot plants. As food sources for both humans and animals, they are economically important and they thrive in a number of habitats, including the tropics.