Aguitarist is tuning a guitar. she hears a low beat every 2.0 seconds. what is the difference between the correct frequency and measured frequency of the guitar string?

A "beat" is an audible period that arises due to a difference wave between two sine oscillations. With that, if the guitarist hears 2 beats per second, i.e., at a frequency of 1/2s = 0.5 Hz, the difference between the guitar and the reference (a tuner tool) is 0.5Hz. This could be due to the guitar being 0.5Hz under the reference, or 0.5Hz above the reference.

positive reinforcement? ?

i think the answer is a, hope this : )