The critical radius of insulation for cylindrical pipe is (where, h_i = heat transfer coefficient at inside of the pipe )

ANS:A - K/h₀

The critical radius of insulation for a cylindrical pipe is the radius at which the heat transfer rate through the insulation is maximized. It occurs when the heat transfer rate due to convection at the inside surface of the pipe (hi​) is equal to the heat transfer rate due to conduction through the insulation (K). The critical radius of insulation is given by: crit=ℎrcrit​=hi​K​ So, the correct answer is: hi​K​