Nusselt number (NN) is given by

Answer: Option A

Nusselt number (N_N) is the ratio of convective to conductive heat transfer at a boundary in a fluid.
A Nusselt number of value one represents heat transfer by pure conduction.
Nusselt number (N_N) is given by $${\text{N}}_{\text{N}}=\frac{\text{h}l}{\text{k}}$$