The purpose of providing a 'catchall' in the vapor line of an evaporator is to

ANS:D - arrest the entrained liquid

The purpose of providing a 'catchall' in the vapor line of an evaporator is typically to vent the non-condensible gases. Non-condensible gases such as air or other gases can accumulate in the vapor space of the evaporator during operation. These gases can hinder the evaporation process by occupying space that should be filled with vapor and can also reduce the efficiency of heat transfer. Therefore, a catchall or venting mechanism is provided to remove these gases from the system, ensuring optimal performance of the evaporator.