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

Hint:Guard cells are plant cells that are present on the epidermis of leaves and stems and help to control the gaseous exchange of plants. They are always present as a pair surrounding the stomatal pore.

Complete answer:
The major function of the guard cell is to regulate the opening and closing of the stomata. The stomatal pore is generally closed unless conditions promote their opening. Conditions such as high light intensity or high humidity activates a proton pump that pumps $H^+$ ions out of the guard cells. This makes the cell electrically negative which in turn activates potassium gated channels that pump $K^+$ ion into the cell.
Malate ions are also produced and as a result the solute concentration of the guard cell increases which results in movement of water molecules inside the cell due to osmosis. This leads to the opening of the stomatal pore since the guard cells bow apart from one another due to increase in turgor pressure.
When the stomatal pore opens carbon dioxide enters into it which is required in photosynthesis and oxygen produced as a product diffuses out through it. When stomatal pore is open it also causes loss of water by evaporation which is known as transpiration.

Hence the correct answer is option (B) Transpiration.

Note:Transpiration is the process by which water in vapour form is released from plants, transpiration in controlled amounts helps in cooling of the plant but excessive transpiration leads to wilting of plants. So the opening and closing of the stomata should be tightly regulated to avoid.