B

Suppose that the separation between two speakers A and B is 3.90 m and speakers vibrate in phase. They play the same 148-Hz tone and the speed of sound is 343 m / s. An observer is sitting at a position just in front of the B speakers so that his line of sight extension of B is perpendicular to the imaginary line between A and B. What is the greatest possible distance between speaker B and the observer, so he observes destructive interference?

For destructive interference distance between the speaker both must differ from λ / 2; 3λ/2...etc

Now, for a frequency of 148hz wavelength is 343/148 = 2.318m

Now, the distance to the speaker B is x and the distance A would be sqrt (x ^ 2 + 3.5 ^ 2)

then sqrt (x ^ 2 + 3.5 ^ 2) - x = 2.318 / 2 = 1.159

if x ^ 2 + 3.5 ^ 2 = (1.159 + x) ^ 2 .... if x ^ 2 + 3.5 ^ 2 = x ^ 2 + 2 * 1159 ^ 2 + 1.159x

simplify 3.5 ^ 2 = 2.318x + 1343

So x = (12.25 in 1343) / 2318 = 4.71m

With these settings you will never encounter destructive interference.

Both are in phase at a similar frequency, so that you can never have destructive interference.