Arrange the different regions of a flame in increasing order of temperature.

The temperature is maximum in the outermost region as it gets maximum oxygen supply and complete combustion takes place. Temperature is the least in the innermost because there is no combustion in that zone. The temperature of the middle zone is in between that of the outer zone and the inner zone because of partial combustion.

Therefore the correct order of temperature is Inner region <  Middle region < Outermost region.