ANS:C - zero

In a parallel flow heat exchanger, if the outlet temperature of the hot and cold fluids are the same, then the log mean temperature difference (LMTD) is zero. The LMTD is calculated as the logarithmic average of the temperature differences between the hot and cold fluids at the inlet and outlet. When the outlet temperatures of the hot and cold fluids are the same, both temperature differences become zero, resulting in a zero LMTD. Therefore, the correct option is: zero