Guttation is mainly due to 

Guttation is the process of loss of water from plant infact, mostly from margins of leaves. It occurs when transpiration rate is very low as compared to rate of water absorption. Due to this root pressure is developed and water is pushed out through specialized pores at vein endings called hydathodes. Each hydathode is made up of group of loosely arranged achlorophyllous or colourless and parenchymatous cells called epithem. These cells help in absorbing a good percentage of inorganic and organic solutes present in the exudate. Thus guttation is due to the positive pressure in the xylem developed due to the root pressure.

So, the correct answer is option A.