Where is the boundary layer thickness and $u_{\infty }$ is the free steam velocity?

Question

What is the ratio of displacement thickness to boundary layer thickness for a linear distribution of velocity  $\frac{u}{u_{\infty }}=\frac{y}{\delta }$ in the boundary layer on a flat plate, where is the boundary layer thickness and $u_{\infty }$ is the free steam velocity? 

(a) 0.5     (b) 0.67   (c) 0.75   (d) 0.8