Reason for operating an evaporator in multiple effect is to secure

ANS:C - both (a) and (b).

Operating an evaporator in multiple effect configuration is primarily aimed at achieving both increased steam economy and decreased steam consumption. In a multiple effect evaporator system, the vapor generated in one effect is used to heat the feed in the next effect, thus reducing the need for external heating steam in each successive effect. This cascading effect leads to significant energy savings and improved steam economy compared to single-effect evaporators. Additionally, by reusing the vapor multiple times, the steam consumption is decreased. Therefore, the correct option is: both (a) and (b).