ANS:A - if the ratio of the delivery pressure to the intake pressure exceeds 5:1.

Multistage compression is necessary for high efficiency when the ratio of the delivery pressure to the intake pressure exceeds 5:1. When the compression ratio (ratio of delivery pressure to intake pressure) becomes too high, a single-stage compressor may become less efficient due to factors such as increased heat generation, mechanical stresses, and reduced volumetric efficiency. Using a multistage compressor allows the compression process to be divided into several stages, with each stage having a lower compression ratio, thereby improving efficiency and reducing the workload on each stage. So, the correct answer is: if the ratio of the delivery pressure to the intake pressure exceeds 5:1.