Dumbbell shaped stomata can be traced in:
a. Monocot leaves
b. Dicot leaves
c. Gymnosperms
d. Both (a) and (c)

Hint:
The stomata in plants mediate the gaseous exchange used for synthesis of food and production of oxygen. The monocot plants possess dumbbell shaped stomata while dicots have bean shaped stomata.

Complete answer:
The stomata are pores found in the epidermis layer of leaves, stems, and sometimes on other parts of plants as well. This pore is used for gaseous exchange in the atmosphere. This pore is bordered by a pair of specialized parenchymatous cells known as guard cells. The guard cells are responsible for regulating the size of the stomatal opening. The guard cells, stomatal pore and the subsidiary cells together constitute the stomatal aperture.
The air enters the plant through these openings by the process of diffusion and absorbs the carbon dioxide molecules which are used in the process of photosynthesis to make food. Consequently, oxygen is produced as a by-product. It diffuses out to the atmosphere through the stomatal pores. You may be amazed to know that water vapors also get diffused into the air through stomata. This process is called transpiration.
In vascular plants their number, shape and size vary widely. In dicots, the stomata are kidney-bean shaped stomata which are arranged in an irregular manner on the leaf surface. The number of stomata present on the lower surface is comparatively greater than on the upper surface.

So, the correct answer option A.

Note: In desert plants, the stomata are deep seated in the leaves and photosynthetic stems to minimize the water loss through the pores. That is why desert plants mainly complete their photosynthesis during night time.