Guard cells help in
A. Protection against grazing
B. Transpiration
C. Guttation
D. Fighting against infection

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Hint: Stomata are the tiny openings present on the leaves and even on the undersides of the leaves. Their function is to exchange gases from the outer environment to the inside of the leaf.

Complete answer:
Stomata are surrounded by two guard cells. It opens and closes in response to these cells. The guard cells are sausage shaped cells. Their inner wall is thick. It becomes tender when there is water inside it. The outer wall expands outward, as a result the stomata enlarges.

Now let us analyze the options:

Option A: Many animals move in herds and feed on various plants and bushes. In order to prevent the animals from eating the plant, the stems modify themselves into thorns or spiny spicules. When an animal tries to eat the plant, it prickles them. This is the modification of a stem. Therefore, this is the incorrect option.

Option B: Transpiration is a phenomenon in which the stomatal pores exchange gases from the environment. The opening and closing of these stomatal pores is regulated by guard cells. Thus it function is to help in transpiration. Therefore, this is the correct option.

Option C: Guttation is a process in which the water droplets are released from some vascular plants. It is different from dew. These droplets have both organic and inorganic compounds. Therefore, this is the incorrect option.

Option D: Various fruits and herbs obtained from the plants help to fight infections and prevent diseases but this is not the function of guard cells. Therefore, this is the incorrect option.

Hence, the correct answer is option (B)

Note: Light is one of the main components in opening and closing of guard cells. Change in the pressure of guard cells changes the opening and closing of stomatal pore.
Some of the important functions of transpiration in plants are:
1.It helps in maintenance of cell turgidity and it also helps in cell division
2.It helps in maintenance of water within the plant as water is eliminated from the plant continuously
3.The cooling effect of tree is because of transpiration.