In standard orthographic projection theory, the plane on which the top view (plan) of an object is projected is called the ________ plane. Identify the correct reference plane name used in descriptive geometry.

Introduction / Context:
Orthographic projection uses three principal reference planes: horizontal (HP), frontal (FP), and profile (PP). Each view corresponds to a projection onto one of these planes, and correct naming is essential for communication.

Given Data / Assumptions:

• The view in question is the top view, also called the plan.
• We follow standard descriptive geometry terminology.
• No special projection (auxiliary) is implied.

Concept / Approach:
The top view is obtained by projecting perpendicular to the horizontal plane. The front view projects onto the frontal plane; side views project onto the profile plane. These definitions are consistent across first- and third-angle systems.

Step-by-Step Solution:

Verification / Alternative check:
Consult any standard projection diagram labeling HP, FP, PP: the plan view clearly lies on HP.

Why Other Options Are Wrong:

• Frontal/Profile: correspond to front and side views respectively.
• Base: nonstandard term in this context.

Common Pitfalls:
Confusing “horizontal plane” with “ground line.” The ground line is the intersection of HP and FP in classical constructions, not the projection plane itself.

Final Answer:
Horizontal