Like richard said, you need to review the documentation for what you are trying to do.
Google needs to become your friend:
DocumentFormat.OpenXml.Wordprocessing.DropDownListSelection - Google Search[
^]
DropDownListSelection Class (DocumentFormat.OpenXml.Wordprocessing)[
^]
You really need to learn how to research your issues via google/any other resource as you could have likely saved yourself a lot of time rather than waiting for strangers on the internet to give you those same links.
Now, once you start working on this and should you get stuck somewhere...feel free to come back with a clear explanation of your issue and any relevant code samples for us to replicate your problem.