A constant, stable guide to which all lengths are directed is referred to as a:

• A. standard of length
• B. conversion factor
• C. derived unit
• D. physical quantity

Answer: (A)

Verified step by step guidance

1. Understand the context of the question: it asks for a term that describes a constant, stable reference used to measure lengths.
2. Recall that in physics, a 'standard of length' is a fixed, agreed-upon reference used to define and compare lengths consistently.
3. Recognize that a 'conversion factor' is used to convert between units, not to serve as a stable reference itself.
4. Note that a 'derived unit' is a unit formed from base units, and a 'physical quantity' is a measurable property, neither of which specifically refers to a stable guide for length.
5. Conclude that the term 'standard of length' best fits the description of a constant, stable guide to which all lengths are directed.