Bronze is the material used in the hairspring of measuring instruments.

Explanation:

1. Introduction to hairspring in measuring instruments:
- Hairspring is a vital component in mechanical measuring instruments like watches, chronometers, and other precision instruments.
- It is a thin, coiled spring that controls the oscillations of the balance wheel in these instruments.
- The balance wheel and hairspring together form the regulating organ of the instrument, ensuring accurate timekeeping or measurement.

2. Importance of material selection for hairspring:
- The material used for the hairspring is crucial as it directly affects the performance and accuracy of the measuring instrument.
- The material should possess certain characteristics such as elasticity, stability, and resistance to corrosion and temperature changes to ensure reliable measurements.

3. Material options for hairspring:
a) Copper: Copper is not suitable for hairspring material due to its low elasticity and susceptibility to oxidation and corrosion.
b) Bronze: Bronze is the correct answer as it is widely used in hairsprings.
- Bronze is an alloy primarily composed of copper and tin, along with other elements like zinc, aluminum, or phosphorus.
- It offers excellent elasticity, stability, and resistance to temperature variations, making it an ideal choice for hairsprings.
- Bronze also has good corrosion resistance, enhancing the longevity and reliability of the measuring instrument.
c) Alnico: Alnico is a type of magnet alloy and not suitable for hairspring material.
d) None of these: This option is incorrect as bronze is indeed the material used in the hairspring of measuring instruments.

4. Advantages of bronze in hairsprings:
- Bronze has a high elasticity, allowing the hairspring to flex and oscillate accurately.
- It exhibits minimal thermal expansion, ensuring consistent performance over a wide range of temperatures.
- Bronze is also resistant to corrosion, preventing degradation of the hairspring and maintaining its accuracy over time.
- The alloying elements in bronze can be adjusted to fine-tune the properties of the hairspring for specific applications.

In conclusion, the material used in the hairspring of measuring instruments is bronze. Bronze offers the necessary characteristics of elasticity, stability, and corrosion resistance, ensuring accurate and reliable measurements.