The correct option is (b) °C
To elaborate: In the particular system, whatever unit is assigned to temperature, the same is the unit of LMTD. It is just a special kind of temperature difference. Logarithmic mean temperature difference. LMTD is a logarithmic average of the temperature difference between the hot and cold streams in heat exchanger. Its unit is whatever is the unit of the temperature. °F or °C or Kelvin.