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.
