Can respondents return to complete and incomplete survey?

It is possible to configure your survey this way if you wish.

To setup your survey so that respondents can return and complete it you will need to access your surveys settings.
In the Survey Status tab select the Take Survey Once, then allow participant to edit responses

This option makes the survey behave in the following ways:

Save and return to survey

Now you will need to configure your survey so that a Save button is shown on it. Navigate to the Misc tab and select the 'Show save button?' option

Enable Survey Save button

The survey will now include a 'Save & Exit' button which whenpressed will show the user the Save Message followed by a URL that they can use to return to the survey.