Effect of thermal boundary layer thickness on Nussult number

Question

The temp distribution within the Laminar thermal boundary layer over a heated isothermal flat plate is given by (T – T_{w) / (T$\infty$} – T_w) (3/2) (y/$\delta _{t}$) –(1/2) (y/$\delta _{t}$)³, where T_w and $T_{\infty }$ are the temp of plate and free stream respectively, and ‘y’ is the normal distance measured from the plate. The ratio of Average to the local Nussult number based on the thermal boundary layer thickness $\delta _{t}$ is given by                                      

      (a) 1.33                           (b) 1.5                          (c) 2.0                             (d) 4.64