28 degrees C.
The dew point is the temperature below which the water vapour in a volume of humid air at a constant pressure will condense into liquid water. Condensed water is called dew when it forms on a solid surface. Dew point provides an idea how much moisture there is in the air a high dew point is very wet air and a low dew point is very dry air.
If the air is gradually cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will saturate the air, is called the dew point . If the air is cooled further, some of the moisture will condense. In the given example, while cooling the sample of air though dew is visible at 27 degrees C, theoretically it must have been forming since the satutaion point i.e. 28 degrees C.