For a given length of air column, the maximum wavelength(or minimum frequency) that can produce resonance is known as fundamental mode, and the frequency at this length is referred to as the natural frequency.
Basically, the open end has to be a displacement anti-node and the other closed end is a displacement node. Separation between a consecutive anti-node and a node is $\lambda/4 $. Hence, waves which have wavelength $\lambda$ in an air column of length $l$ will produce resonance if $\lambda/4=l$, $3\lambda/4=l$ or $5\lambda/4=l$ and so on. Therefore the largest possible wavelength that would cause resonance is when $\lambda/4=l$.Wavelengths above this will not be able to resonate.
Furthermore, if speed of sound remains unchanged for different lengths the natural frequency will have an inverse proportionality with the length of the air column.
$l$ is length of air column, v is speed of sound and f is natural frequency.
Now, if we keep the frequency same by using a tuning fork to produce sound waves, and change the length of the air column, then for that particular frequency $f$ there is particular wavelength $\lambda$ associated with it(as speed of sound is constant). For this wavelength there are different lengths$l_1, l_2, l_3....$ and so on which resonate so that $l_1=\lambda/4$; $l_2=3\lambda/4$; $l_3=5\lambda/4$.
The length which would be resonating at natural frequency would be $l_1$.Although, $l_2$ and $l_3$ would also be resonating but not at the length's natural frequency since for $l_2$ and $l_3$ the length is not quarter of the wavelength, which is the condition required for fundamental mode of resonance.