Apical meristem
The correct answer is option 'A', apical meristem. Histogens are components of the apical meristem. Let's understand what histogens and apical meristem are and why histogens are associated with the apical meristem.

Apical Meristem
The apical meristem is a type of meristem located at the tips of the plant's roots and shoots. It is responsible for the primary growth of plants, which includes the increase in length of the roots and shoots. The apical meristem consists of undifferentiated cells that have the ability to divide and differentiate into specialized cells of various plant tissues.

Histogens
Histogens are specific regions within the apical meristem that give rise to different plant tissues. They are responsible for the organization and development of the plant's primary meristems, which eventually differentiate into different types of tissues. Histogens are involved in the formation of the three primary meristems: protoderm, ground meristem, and procambium.

Protoderm
The protoderm is the primary meristem responsible for the formation of the epidermis, which is the outermost layer of cells in plants. It gives rise to various epidermal tissues, such as the protective cuticle, stomata, and trichomes.

Ground Meristem
The ground meristem is the primary meristem responsible for the formation of ground tissues in plants. Ground tissues include parenchyma, collenchyma, and sclerenchyma cells. Parenchyma cells are involved in photosynthesis, storage, and secretion. Collenchyma cells provide support to growing plant parts, and sclerenchyma cells provide mechanical strength and support.

Procambium
The procambium is the primary meristem responsible for the formation of vascular tissues in plants. Vascular tissues include xylem and phloem. Xylem transports water and minerals from roots to shoots, while phloem transports sugars and nutrients from leaves to other parts of the plant.

Conclusion
In conclusion, histogens are components of the apical meristem. They are responsible for the organization and development of the primary meristems, which eventually differentiate into different plant tissues. The three primary meristems formed by histogens are the protoderm, ground meristem, and procambium. The protoderm gives rise to the epidermis, the ground meristem forms ground tissues, and the procambium forms vascular tissues.