Yes, in-line would be the easiest. And you should be able to find it at Lowe's or Home Depot. Most folks would have to have an electrician do either of the other two options.
Just make sure you don't already have a GFCI device at the outlet you're plugging into. It will have 'test' and 'reset' buttons if it's a GFCI.
Just make sure you don't already have a GFCI device at the outlet you're plugging into. It will have 'test' and 'reset' buttons if it's a GFCI.