The cross dial A reads off of the __________.

The cross dial is an instrument used in precision alignment tasks, primarily in the context of machinery installation and maintenance. It is typically utilized in settings where exact measurements are critical for ensuring that parts align accurately for optimal operational performance.

When the cross dial reads off of a fixed machine, it means that it is measuring the position relative to a stationary reference point. This is essential because a fixed reference allows for consistent and repetitive measurements, which helps in identifying any misalignments or deviations in the rotational axis of the machinery being aligned.

In alignment procedures, using a fixed machine as a reference increases the accuracy of the readings taken by the cross dial, thereby facilitating adjustments that ensure the eventual operational efficiency and longevity of the machinery involved.

The other options pertain to different types of systems or concepts in machinery and alignment but do not specifically designate a fixed machine as the reference for readings by the cross dial. Thus, the connection to the fixed machine underscores its role as a critical point of reference in alignment tasks, confirming the correctness of the answer provided.