Missing options: Increasing the wave amplitude. Increasing the wave frequency. Stretching the elastic cord further. Increasing the wavelength.
Answer: Stretching the elastic cord further. In fact, the speed of a wave in a string is given by [tex]v= \sqrt{ \frac{T}{\mu} } [/tex] where T is the tension in the string, and [tex]\mu[/tex] is the linear density. If we stretch the elastic cord further, then the tension of the string T increases, therefore the velocity of the wave v increases as well.
