When more than one rod size is available, the smallest rod is known as the __________ rod.

The term "standard rod" refers to the smallest rod available when multiple rod sizes are offered. In the context of pneumatic systems, the smallest rod typically has certain advantages, such as lower weight and reduced friction, which can contribute to increased efficiency in applications where space is limited. These rods are designed to meet typical operational requirements without the additional features or bulk that larger rods possess.

Other options represent different types of rods that are suited for specific applications. For instance, a miniature rod would be even smaller than the standard, primarily designed for very compact applications, while a compact rod typically aims for a balance of size and strength but may not be the smallest available. Heavy-duty rods are designed for high-load applications, making them larger and more robust than a standard rod. Thus, the "standard rod" serves as the baseline reference for size in this scenario.