The correct answer is option C, that is, regulate water loss.
Guard cells refer to the cells enclosing each stomata. They assist in monitoring the rate of transpiration by closing and opening the stomata. The guard cells possess the tendency to monitor the closing and opening of stomata by changing shape. The shape of the guard cells modifies on the basis of the concentration of potassium ions and water found in the cells themselves.
The stomatal pores get closed when carbon dioxide is no longer needed for the process of photosynthesis. The guard cells swell when movement of water takes place inside these pores, and thus, the opening of stomatal pores occurs, and as water moves out, the guard cell closes. Thus, guard cells play an essential role in regulating water loss.
