Which of the following increases the speed of waves in a stretched elastic cord?

Missing options:
Increasing the wave amplitude.
Increasing the wave frequency.
Stretching the elastic cord further.
Increasing the wavelength.

 Stretching the elastic cord further.
In fact, the speed of a wave in a string is given by

where T is the tension in the string, and  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.