How do I create this summary based on a multi-select text field?

I think you would nee to have a formula numeric field per Choice.

The field called [Option 1] would be like

var text ChoicesInText = ToText([Field]);

If(Contains($ChoicesInText, "Option 1"),1,0)

Same for Option 2

Then run your report to Total each of the [Option x] fields.