Answer
$v_{\circ}cos\theta$
Work Step by Step
We know that $Average \space velocity=\frac{total \space displacement}{total\space time \space taken}$.........eq(1)
As total displacement is equal to the range in the given scenario and $range=\frac{v_{\circ}^2sin2\theta}{g}$
and total time taken is equal to time of flight and time of flight $=\frac{2v_{\circ}sin\theta}{g}$
We plug in the known values in eq(1) to obtain:
$Average \space velocity=\frac{v_{\circ}^2 sin2\theta}{g}\times \frac{g}{2v_{\circ}sin\theta}$
$\implies Average \space velocity=\frac{v_{\circ}^2 2sin\theta cos\theta}{g}\times \frac{g}{2v_{\circ}sin\theta}$
This simplifies to:
$Average \space velocity=v_{\circ}cos\theta$
