Check the documentation
https://closedxml.codeplex.com/wikipage?title=Data%20Validation
https://closedxml.codeplex.com/wikipage?title=Data%20Validation
ws.Cell("C1").Value = "Yes";
ws.Cell("C2").Value = "No";
ws.Cell("A5").DataValidation.List(ws.Range("C1:C2"));