A 256 Hz tuning fork is resonating in a closed tube on a warm day when the speed of sound is 346 m/s. What is the length of the closed tube?

A. 1.35 m

B. 0.74 m

C. 0.34 m

D. 0.18 m

Assuming our "closed tube" is closed at only one end, then

v = fλ = f*4L/n

where "n" is the harmonic number. So

L = nv / 4f = n*346m/s / 4*256Hz = n*0.38 m

Since the only option in your list that is an integer multiple of 0.38 m is 1.35 m

I'd say that we're hearing the fourth harmonic.

answer is

A. 1.35 m