The strain-stress plot for materials $A, B, C$ and $D$ is shown in the figure. Which material has the largest Young's modulus ?

Young's modulus ($Y$) is defined as the ratio of stress to strain: $Y = \frac{\text{stress}}{\text{strain}}$.
In the given graph, the y-axis represents strain and the x-axis represents stress.
The slope of the line in a strain-stress graph is given by $m = \frac{\text{strain}}{\text{stress}}$.
Therefore, $Y = \frac{1}{\text{slope}}$.
To have the largest Young's modulus, the material must have the smallest slope in the strain-stress plot.
From the figure, line $C$ has the smallest angle with the stress axis (x-axis), meaning it has the minimum slope.
Since $Y \propto \frac{1}{\text{slope}}$, material $C$ has the largest Young's modulus.