The only paramagnetic one is selenium.
A paramagnetic species will have an electron configuration that shows unpaired electrons. Unpaired electrons are what will cause the respective atom (or ion) will be attracted to a magnetic field.
If the opposite is true, i.e. a species has no unpaired electrons in its electron configuration, than that species will be diamagnetic.
So, all you have to do is write the electron configurations (I'll use noble gas shorthand notation) for all the species listed.
Start with the chlorine ion,
Since a noble gas configuration has no unpaired electrons, it will result in a diamagnetic species.
The same idea applies for the barium and scandium cations as well. Neutral barium is
If you remove the outermost two electrons, you'll get the noble gas configuration of xenon. Once again, this will result in a diamagnetic species.
This brings us to selenium,
Now, you'd be tempted to say that selenium has no unpaired electrons either, but you'd be wrong. Here's why
Selenium actually has 2 unpaired electrons in the 4p-orbital, which makes it paramagnetic.