Which type of compressor has multiple pistons and cylinders but is considered single stage?

The correct choice is based on the understanding of how a single-stage compressor operates with multiple pistons and cylinders. In this scenario, a compressor can still be classified as single-stage if each cylinder discharges directly into the receiver. This means that all the pistons compress the air in a single stroke to the same pressure before it enters the storage tank, or receiver, without any intermediate stage of compression. The configuration allows for a straightforward process where the air undergoes a single compression boost, which is essential to maintaining efficiency and simplifying the design.

The other options present circumstances or characteristics that do not inherently define the compression stage. While a compressor that runs on a gas engine, operates under a vacuum, or is portable might have different operational features, these aspects do not determine whether the compressor is single-stage or multi-stage. The defining characteristic lies in the design and how the air is processed through the cylinders into the receiver, confirming that the functionality remains consistent with a single-stage compressor despite having multiple pistons.