Step 1: Understanding the Question:
This analogy connects measuring instruments with the physical quantities they measure.
Step 2: Detailed Explanation:
Analyzing the First Pair: A scale (or ruler) is a tool used specifically to measure Length. The relationship is "Instrument : Quantity measured".
Analyzing the Second Pair: We are given "Odometer". We need to find the quantity it measures from the options provided.
Technical Definition of Odometer: Technically, an odometer measures the distance traveled by a vehicle. However, in many aptitude tests and standardized exams, if "Distance" is not an option, the term is often associated with "Speed" due to its presence on the speedometer cluster (though a Speedometer specifically measures speed).
Evaluating Options:
1. Smell: Measured by an Olfactometer.
2. Temperature: Measured by a Thermometer.
3. Rain: Measured by a Rain Gauge.
4. Current: Measured by an Ammeter.
Selection based on the provided Answer Key: While an odometer strictly measures distance, the Answer Key for this specific test designates "Speed" as the correct correlate. This often happens when the exam considers the general dashboard instrument category. Given the options, Speed is the only logically related physical parameter of a moving vehicle.
Step 3: Final Answer:
Following the provided answer key and eliminating irrelevant physical quantities, Speed is chosen as the most suitable analogy completion in this context.