posted by Joanna .
Which one of the following frequency can set the string into resonant vibration?
If a guitar string has a fundamental frequency of 500Hz
It has to be an integral multiple of 500 Hz. Only one of those frequencies meets that requirement.
what does that mean?
The ratio of a resonant frequency to 500 Hz (the fundamental) must be an integer. That is only true for one of the four choices.
That is what it means.