In Biot number, the characteristic length used is the ratio of the __________ of the solid.

Answer: Option A

The Biot number (Bi) is a dimensionless quantity used in heat transfer.

It represents the ratio of the internal thermal resistance within a solid to the external thermal resistance between the solid and its surrounding fluid.

In the Biot number calculation, the characteristic length is defined as the ratio of the volume of the solid to its surface area.

This characteristic length helps determine whether temperature gradients within the solid are significant compared to those at the surface.