Correct option is (b) T∞

The explanation is: Let X be the unknown new temperature at the wall surface. If the heat transfer coefficient at the surfaces of a wall is assumed to be h, then the heat interaction at the surfaces of the wall is evaluated by equating the conduction heat transfer to the convection heat transfer, i.e.,

kATx = hA(X – T∞)

\(\frac{-kT_x}{h}\) = X – T∞

Given h = ∞

\(\frac{-kT_x}{\infty}\) = X – T∞

0 = X – T∞

X = T∞.