Verification of truth: A pen necessarily has which of the following? Choose the constituent without which it cannot function as a pen.

Introduction / Context:
We must identify the indispensable element of a writing pen, i.e., the feature without which it cannot serve its essential purpose: depositing writing medium on a surface.

Given Data / Assumptions:

• “Pen” refers to ink-based writing instruments (ballpoint, gel, fountain, rollerball, markers).
• Accessories (caps, holders) vary widely by design.

Concept / Approach:
All pens, regardless of tip or body design, deliver ink to the writing surface. Other components, while common or convenient, are not universal necessities.

Step-by-Step Solution:

Verification / Alternative check:
Across subclasses of pens, ink (or ink-equivalent dye solution) is the invariant requirement for writing marks.

Why Other Options Are Wrong:

• Tube, Cap, Holder: design-dependent, not universal.
• None of these: incorrect because ink is necessary.

Common Pitfalls:
Mistaking typical housings for functional essence.

Final Answer:
Ink