Assuming the air pressure, humidity, and all other factors are the same for all of these temperatures, it would be the lowest one (-5 Celsius). Lower temperatures allow the sound waves to travel through it faster because the air molecules are slightly more dense. The more dense the substance is, the faster sound travels through it. Absolute zero would make it travel the fastest. Remember that air pressure also matters too, because the more pressurized it is, the more dense it is, and the more dense it is, the faster that sound travels. Humidity, altitude (air pressure again), and other factors similar also affect it. Plain and simple, the answer is -5.
At 0°C is ?0 = 1.293 kg/m3, Z0 = 428 N·s/m3, and c0 = 331 m/s
At 15°C is ?20 = 1.225 kg/m3, Z20 = 417 N·s/m3, and c20 = 340 m/s
At 20°C is ?20 = 1.204 kg/m3, Z20 = 413 N·s/m3, and c20 = 343 m/s
At 25°C is ?20 = 1.184 kg/m3, Z25 = 410 N·s/m3, and c25 = 346 m/s
Air density or density of air ?, air impedance Z, speed of sound c
The speed of sound in air is determined by the air itself and is not
dependent upon the amplitude, frequency, or wavelength of the sound.
For an ideal gas the speed of sound depends only on the temperature and
is independent of gas pressure. This dependence also applies to air, in
good approximation and can be regarded as an ideal gas.