ANS:B - painting the surface white (with aluminium paint).

To reduce the heat transfer by radiation from a mild steel surface by reducing its emissivity, the best approach would be to give the surface a mirror finish. Emissivity is a measure of how effectively a surface emits thermal radiation. A mirror finish surface typically has low emissivity because it reflects most of the incident radiation rather than absorbing and emitting it. By giving the surface a mirror finish, the emissivity can be reduced, which in turn reduces the amount of heat transferred by radiation. Therefore, the correct option is: giving the surface a mirror finish.