Verification of truth (clarified): Consider a traditional incandescent electric bulb. Which of the following does it necessarily have?

Introduction / Context:
The original item used “bulb,” which today could include LEDs and CFLs. Applying the Recovery-First Policy to remove ambiguity, we explicitly interpret it as a traditional incandescent electric bulb. We then identify the indispensable internal part.

Given Data / Assumptions:

• Device: incandescent bulb (classic type).
• We look for a structural necessity, not a state (like being lit) or external condition (like current being supplied).

Concept / Approach:
Incandescent bulbs operate by passing current through a high-resistance filament that heats to incandescence and emits light. Without a filament, it cannot function as such a bulb.

Step-by-Step Solution:

Verification / Alternative check:
Contrast with LED “bulbs” that have no filament; that is why we clarified scope to incandescent.

Why Other Options Are Wrong:

• Light: not a part; not always present.
• Glass: common but not logically indispensable as the operating principle is the filament.
• Current: operating condition, not a component.
• None of these: incorrect because filament is necessary.

Common Pitfalls:
Equating typical materials (glass) or operating states (light/current) with essential internal mechanism (filament).

Final Answer:
Filament