To make radio button selected by default, firstly create a new or open old Contact Form.
Then create new field – Radiobuttons List – http://prntscr.com/doevg2
Add options.
Add Name the created option you need to Default Value and Save changes.
Screenshot http://prntscr.com/doj3ow
So it will look like: screenshot http://prntscr.com/doj3yh
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