Refer to figures. 
Aircraft icing can be divided into two general types based on whether it affects power production or aerodynamics. Induction icing affects engine power and includes when ice forms in the carburettor venturi. It can occur when the outside air temperature is between -10°C and +30°C and with a high relative humidity.

Carburettor icing is particularly common, as the air cools down significantly inside the carburettor, not only due to the decreased static pressure in the venturi section (decreasing the temperature adiabatically) but also the evaporation of the fuel inside the carburettor, which requires latent heat input, thereby cooling the air. It is worse when the throttle is low, thereby causing the throttle butterfly valve to have only a small opening, further increasing the venturi effect.

As you can see by the graph above, the worst icing happens between about -1ºC and +15°C, which is very close to the correct answer given here.

Too warm, and the air won't cool down enough in the carburettor to form ice. Too cold, and there isn't usually enough water vapour in the air to develop carburettor icing.