The correct answer is option 'A': Sepals, petals, stamens, carpels.

- Sepals: The sepals are the outermost whorl of the flower, located at the base of the flower. They are usually green in color and protect the developing flower bud. They are collectively called the calyx.

- Petals: The petals are the second whorl of the flower, located inside the sepals. They are often brightly colored and attract pollinators like insects and birds. They are collectively called the corolla.

- Stamens: The stamens are the third whorl of the flower, located inside the petals. Each stamen consists of a filament and an anther. The anther produces pollen grains, which contain the male gametes or sperm cells. The stamens are collectively called the androecium.

- Carpels: The carpels are the innermost whorl of the flower, located in the center. Each carpel consists of an ovary, a style, and a stigma. The ovary contains one or more ovules, which contain the female gametes or egg cells. After fertilization, the ovary develops into a fruit. The carpels are collectively called the gynoecium.

These four whorls together make up the complete flower structure. The sepals and petals are non-reproductive structures and are involved in protecting and attracting pollinators, respectively. The stamens and carpels are reproductive structures and are involved in the production of gametes and subsequent fertilization.

It is important to note that not all flowers have all four whorls. Some flowers may lack one or more whorls, depending on the species. For example, some flowers may lack petals or have modified sepals that resemble petals. The arrangement and characteristics of these whorls can vary greatly among different plant species.