What is the shape of the graph of stress $\to$ strain up to the elastic limit?

The elasticity of Invar:

Hooke's law defines:

$Assertion :$ Strain causes the stress in an elastic body.
$Reason :$ An elastic rubber is more plastic in nature.

Anvils made of single crystals of diamond,with the shape as shown in the figure,are used to investigate the behavior of materials under very high pressures. The flat faces at the narrow end of the anvil have a diameter of $0.50 \; mm$,and the wide ends are subjected to a compressional force of $50,000 \; N$. What is the pressure at the tip of the anvil?

Explain Hooke's Law.