ANS:B - slightly concave upward.

The adiabatic saturation curve represents the relationship between the temperature and the humidity ratio of a vapor-gas mixture when the mixture undergoes an adiabatic process (i.e., no heat exchange with the surroundings). This curve is typically not a straight line. It generally exhibits a slight concave upward shape. This curvature arises due to the fact that as the temperature of the mixture increases, the vapor content (humidity ratio) of the mixture increases as well, but at a decreasing rate. This is because as the temperature rises, the air's capacity to hold moisture increases, but the rate at which moisture is added to the air may not increase as rapidly. Therefore, the correct answer is: slightly concave upward.