ANS:A - increases

As the reflux ratio increases, the slope of the operating line for the rectifying section in a distillation column increases. The operating line in the rectifying section represents the equilibrium relationship between the vapor and liquid phases as they ascend the column. The reflux ratio is defined as the ratio of the reflux flow rate to the distillate flow rate. Increasing the reflux ratio means that more liquid is returned to the top of the column relative to the amount of distillate being withdrawn. This increases the liquid flow rate in the rectifying section. Since the slope of the operating line is determined by the ratio of vapor flow rate to liquid flow rate, an increase in the liquid flow rate (due to increased reflux ratio) will lead to a steeper slope of the operating line. Therefore, as the reflux ratio increases, the slope of the operating line for the rectifying section increases.