Which of the following represents the three datum reference frames?

The concept of datum reference frames is fundamental in Geometric Dimensioning and Tolerancing (GD&T), serving as a reference point for measurement and ensuring parts are manufactured and assembled within specified tolerances. The three datum reference frames are designated as primary, secondary, and tertiary datums.

The primary datum is the main reference that establishes the initial alignment for the part; it is typically the most important surface or axis from which measurements are taken. The secondary datum provides a second reference point, usually at a right angle to the primary datum, which helps to establish the next level of orientation. Finally, the tertiary datum is used to further define the location and orientation by providing a third point of reference, often at a right angle to both the primary and secondary datums.

This systematic approach, using three distinct levels of datums, ensures that parts can be accurately and consistently measured, regardless of the complexity of the design. The other groups of terms do not accurately reflect the structured system of datum reference frames established by GD&T principles.