• ARatio of internal thermal resistance of a solid to the boundary layer thermal resistance
• BRatio of the rate at which internal energy is advected to the rate of conduction heat transfer
• Non-dimensional temperature gradient
• DNon-dimensional velocity gradient multiplied by Prandtl number

By definition, Nu=hL/k, and from boundary-layer analysis h=−k(∂T/∂y)|surface/(Ts−T∞), so Nu=−(∂T*/∂y*)|surface — a non-dimensional temperature gradient evaluated at the surface. (Option A is instead the definition of the Biot number, not Nusselt number.)