Substance having very small co-efficient of linear expansion, among the following is

In case of invar , the potential well of the of potential energy (u) v.s. seperation (r) between two atoms is nearly symmentrical about a vertical line through equilibrium separation as shown in the figure.So the mean separation between two atoms remains almost the same.

Hence option (4) is right.

**For invar steel coefficient of linear expansion is very small because it is an alloy.