If the radius of a circle passing through all vertices of a rectangle is $4cm$, then the length of the diagonal is

The correct option is B: $8cm$

Given, a circle is passing through all the vertices of the rectangle $ABCD$.

In a cyclic quadrilateral, sum of opposite internal angles is ${180}^{\circ }$.

Since, $\angle BAD=\angle BCD={90}^{\circ }$

$\widehat{ADB}$ and $\widehat{BDC}$ are semicircles as angle in a semi-circle is ${90}^{\circ }$.

$BD$ is the diameter of the circle and also the diagonal of the rectangle.

$BD=2\times \left(\text{radius}\right)=2\times 4=8cm$