Line conventions: An extension line in technical drawing is a thin, dark, solid line that extends from a feature to which a dimension refers, without touching the arrowheads. Confirm this definition.

Introduction / Context:
Standardized line types improve readability of engineering drawings. Distinguishing extension lines from dimension lines, centerlines, hidden lines, and other conventions is foundational knowledge for drafting and interpretation.

Given Data / Assumptions:

• We use common standards (e.g., ASME Y14) for line weights and types.
• Extension lines connect the geometry to dimension lines but are not themselves dimension lines.
• Visibility and consistency are key goals.

Concept / Approach:
An extension line is drawn as a thin, dark, solid line that starts slightly away from the object edge, extends past the dimension line, and terminates cleanly. It identifies the exact points on the geometry being measured. Dimension lines carry arrowheads and the numerical value; extension lines do not carry arrowheads and should not coincide with visible object lines. This visual grammar reduces clutter and misreading.

Step-by-Step Solution:

Verification / Alternative check:
Compare against a drafting legend or template that lists line types: visible (thick), hidden (short dashes), centerlines (long-short), phantom (long-short-short), and extension/dimension (thin solid). The extension line description will match the rule above.

Why Other Options Are Wrong:

• Hidden lines are dashed and typically thin, not continuous; they represent obscured edges.
• Centerlines have alternating long and short dashes and mark axes or symmetry.
• Phantom and chain lines have specialized uses, not for basic dimension references.

Common Pitfalls:
Letting extension lines touch arrowheads incorrectly; drawing them too thick; failing to offset a small gap from the object edge; aligning extension lines over object lines causing confusion.

Final Answer:
Correct: thin, dark, solid line extending from the feature for dimensioning