I had a similar problem with trying to find the parent ID of a specific field in a 2013 workflow
I solved mine in the following way (although for me I wanted to know the parent ID of the field I was running the workflow on so was also filtering)
siteurl/_api/web/lists/GetByTitle('ListName')/items?$select=Id,Title,ParentID/ID&$expand=ParentID&$filter=ID eq '[%CurrentItem:ID%]'
The trick was when trying to extract the ParentID to use
this finally returned the parent ID of the item I was running my workflow on.
The snapshot above occurs after using a "Call" action in a previous workflow step, which retrieves information from an HTTP Web Service. If you are new to working with Web Services in SharePoint 2013 Workflows using SharePoint Designer 2013, the following site can be referenced as a guide: link
If your workflow is limited in scope to a single Task List, one could consider using the "getbyid" which calls that list by its GUID. E.g.,
https:// <Top level Site URL> /_api/web/lists/getbyid(' <List GUID with Dashes in apostrophes> ')/items?$select=Id,Title,ParentID/ID&$expand=ParentID&$filter=ID eq '[%Variable: currentItem%]'
Finally, just a quick bit of additional context related to the formatting of the $select= portion of our query. ParentID is follwed by a "/" and "ID" because the ParentID field is actually a lookup field containing another item ID from our task list. The $expand= portion of our query further handles this Lookup Field behavior. See here for additional context.