The correct answer is $\boxed{\text{B) 0.25 Hz}}$.
The natural frequency of a vibrating reed is determined by its length, mass, and tension. The natural frequencies of two adjacent reeds will be very close together, but not exactly the same. This is because the reeds will have slightly different lengths, masses, and tensions. The difference in natural frequencies between two adjacent reeds is called the “reed spacing”. The reed spacing is typically chosen to be a small fraction of the fundamental frequency of the reed, such as 0.25 Hz. This ensures that the reeds will vibrate in unison when the meter is tuned to the correct frequency.
Option A is incorrect because 0.1 Hz is too small a difference to be noticeable. Option C is incorrect because 0.5 Hz is too large a difference to be practical. Option D is incorrect because 1.5 Hz is too large a difference to be accurate.