Answer
Choice D.
Work Step by Step
The speed of a transverse wave in a string measures how fast the wave disturbance travels along (parallel to) the string. As shown in equation 11-13, it depends on the tension in the string and the linear mass density.
It does not depend upon the amplitude, frequency, or wavelength.
Stretching the elastic cord increases the tension and decreases the mass per unit length, so this will increase the wave speed along the cord.