Final answer:
The speed v of a periodic wave is calculated using the equation v = λ/T, where λ is the wavelength and T is the period of the wave.
Step-by-step explanation:
The speed v of a periodic wave traveling in a string is determined by the distance it travels over a given time period. Given that a wave travels a distance of one wavelength λ in a time interval of one period T, the wave's speed can be calculated using the equation v = λ/T. None of the provided options A, B, C, E, Y²T are correct; the correct formula is represented by option D, v = λ/T.