Transpiration mainly occurs through:

The correct option is C. Stomata.

Stomata are minute aperture structures on plants found typically on the outer layer of the leaf called the epidermis. They consist of two specialized cells, called as guard cells that surround a tiny pore called a stoma. Less than 1% of the water reaching the leaves is used in photosynthesis and plant growth and the excess water needs to be removed. The excess water is lost through the stomata in the leaves by a process called transpiration.

A very small amount of transpiration also occurs in the cuticles and the lenticels. But it is very low compared to the rate of transpiration through the stomata.