ANS:A - increases

With a decrease in throughput compared to the design capacity for a bubble cap distillation column, its efficiency generally decreases. Here's why:

1. Operating Conditions: Distillation columns are designed to operate optimally at certain throughput rates. When the throughput decreases below the design capacity, the operating conditions within the column may deviate from the optimal conditions. This deviation can lead to reduced efficiency.
2. Changes in Vapor-Liquid Flow: At lower throughput rates, the flow of vapor and liquid through the column may be affected. This can alter the distribution of the liquid and vapor phases across the trays, affecting the efficiency of mass transfer.
3. Residence Time: With reduced throughput, the residence time of the vapor and liquid within the column may increase. This can affect the equilibrium between the phases and the overall separation efficiency.
4. Mass Transfer: Mass transfer efficiency, which is crucial for separation in a distillation column, may decrease when operating below design capacity. This can result in poorer separation of the components.

While it's possible for certain factors to mitigate the efficiency decrease or for specific designs to behave differently, in general, a decrease in throughput compared to the design capacity tends to lead to reduced efficiency in bubble cap distillation columns.