What is the most appropriate opposite of "Select All"?

As far as I have seen in software, there is usually a Select All and Clear All (for when there are some/all selected categories) but it would also be appropriate to use Deselect All instead of Clear All.

Select None would be so that (in software terms) that NO categories are to be selected at all. Unless you assign it a specific language code (for example) to make it do the same as Clear/Deselect All.

Clear Selection, as @ManishGiri mentions, is the most common choice among web forms.

Other options include: Reset Form, Clear All, Reset, Unselect