Explanation:
When a stream of moist air is passed over a cold and dry cooling coil, the air stream undergoes a cooling process. The cooling coil is at a lower temperature than the entering moist air, causing heat transfer from the air to the cooling coil. As a result, the air stream gets cooled.

Constant Wet Bulb Temperature:
The wet bulb temperature is a measure of the lowest temperature that can be reached by evaporative cooling. If the wet bulb temperature remains constant during the cooling process, it means that the air is being cooled by evaporative cooling. However, in this case, no condensation is taking place, so the cooling process is not based on evaporative cooling. Therefore, the air stream is not cooled along the line of constant wet bulb temperature.

Constant Dew Point Temperature:
The dew point temperature is the temperature at which the air becomes saturated and condensation starts to occur. If the dew point temperature remains constant during the cooling process, it means that the air is being cooled to a temperature below its dew point temperature. However, in this case, no condensation is taking place, so the air stream is not cooled to a temperature below its dew point temperature. Therefore, the air stream is not cooled along the line of constant dew point temperature.

Constant Relative Humidity:
Relative humidity is the ratio of the amount of moisture present in the air to the maximum amount of moisture the air can hold at a given temperature. If the relative humidity remains constant during the cooling process, it means that the cooling process is not affecting the moisture content of the air. However, in this case, the cooling process is causing the air to lose heat, which results in a decrease in its temperature. Therefore, the relative humidity of the air stream will increase as the air gets cooled. Hence, the air stream is not cooled along the line of constant relative humidity.

Constant Enthalpy:
Enthalpy is a measure of the total heat content of a substance. If the enthalpy remains constant during the cooling process, it means that no heat is being added or removed from the air stream. However, in this case, the cooling coil is at a lower temperature than the entering air, causing heat transfer from the air to the cooling coil. Therefore, the enthalpy of the air stream decreases as it gets cooled. Hence, the air stream is cooled along the line of constant enthalpy.

Therefore, the correct answer is option 'B' - constant dew point temperature.