Correct answer: 510 Hz
The beat frequency is the difference between the tuning fork's frequency and the piano string's frequency. Initially, the beat frequency is 4 Hz, so the piano string's frequency could be either 512 + 4 = 516 Hz or 512 - 4 = 508 Hz. When the tension is increased, the frequency of the string increases, reducing the beat frequency to 2 Hz. This means the initial frequency must have been 510 Hz (512 - 2).