ANS:C - nickel

The principal alloying element in Elinvar, which is used for making hair springs for watches, is nickel. Elinvar is a type of nickel-iron alloy known for its low coefficient of thermal expansion (the change in length or volume of a material in response to a change in temperature) and its ability to maintain a constant modulus of elasticity over a wide temperature range. These properties make Elinvar ideal for use in precision instruments such as watches, where maintaining accurate timekeeping despite temperature variations is crucial. In addition to nickel, Elinvar may also contain small amounts of other elements such as chromium and cobalt to further enhance its mechanical properties and stability. However, nickel is the primary alloying element responsible for the unique properties of Elinvar alloys.