Analogy — identify the instrument-to-measurement relation parallel to Thermometer : Temperature.

Introduction / Context:
The pair “Thermometer : Temperature” represents an instrument and the physical quantity it measures. We must choose another pair where the first term is an instrument and the second is its measurement output or quantity.

Given Data / Assumptions:

• Thermometer measures temperature.
• Seek: instrument → what it measures.
• Medical and scientific instruments are valid domains.

Concept / Approach:
Evaluate each option by asking: is the first term a recognized instrument, and is the second the parameter it records? “Cardiograph” (e.g., ECG) records cardiac activity such as heart rate or electrical patterns, satisfying the same instrument-to-measurement relation.

Step-by-Step Solution:

Verification / Alternative check:
Substitute in a sentence: “A cardiograph measures heart rate,” analogous to “A thermometer measures temperature.” The parallel is exact.

Why Other Options Are Wrong:

• Millimeter : Scale — unit and tool reversed and mismatched.
• Length : Breadth — two dimensions, not instrument-to-quantity.
• Solar Energy : Sun — source and product, not measurement.

Common Pitfalls:
Choosing related science words without preserving the instrument→quantity direction. Focus on function, not mere association.

Final Answer:
Cardiograph : Heart rate