What is the wavelength of a 250 hz pure tone

Wavelength = (speed) / (frequency)

So you see, the wavelength is also going to depend on the wave's speed, which you didn't tell us.

-- If the wave is traveling in iron, its speed is about 5,120 m/s.

Wavelength in iron = (5,120 m/s / 250 Hz)

Wavelength in iron =  20.48 meters

-- If the wave is traveling in water, its speed is about 1,480 m/s .

Wavelength in water = (1,480 m/s / 250 Hz)

Wavelength in water =  5.92 meters

-- If the wave is traveling in air, its speed is about 340 m/s .

Wavelength in air = (340 m/s / 250 Hz)

Wavelength in air =  1.36 meters

if they have a finite resultant