Main alloying element in Elinvar used in precision instruments, hair springs for watches etc. is

ANS:B - nickel

The main alloying element in Elinvar used in precision instruments, hair springs for watches, etc., is Nickel. Elinvar is a type of alloy known for its high elasticity and low coefficient of thermal expansion, making it particularly suitable for applications where precise and consistent performance under varying conditions is required, such as in timekeeping mechanisms. It typically consists of iron-nickel-chromium alloys with small additions of other elements to achieve its desired properties.