how to share the ontology. what option must be used?
There are many ways to share an ontology. The ontology is represented as an OWL file so the easiest is just to send the OWL file to someone else. When Lorenz and I have been encouraging you to share your ontology as well as the screen prints with your questions that's what we are talking about, you can take the OWL file and attach it to an email just like you would attach a PDF or any other file. If your OWL file is large you probably want to Zip it first but most OWL files by new users aren't very large.
So just go into the Desktop version of Protege and select Save. It will prompt you for the format to use. RDF/XML is the default which is probably the way to go if you are a new user. You can then send that OWL file to anyone else who has the desktop version of Protege and they can look at it.
Of course some times you want to share an ontology with people who you don't expect to understand much about Protege or OWL. In that case it often can be easier to just put your ontology into Web Protege and give them the URL. They still need to create a Web Protege account but it's easier than requiring them to download a new tool onto their desktop. I use that a lot because many of the people I'm trying to work with don't know much about ontologies and I want them to look at the way I'm modeling the domain and not care that much about OWL so it's easier to just have them login to Web Protege. Also, Web Protege is good for collaboration because it allows multiple users to edit and comment and you can control whether other users can just comment or can also edit.
Uploading to Web Protege is very straight forward, just create a new project and it will prompt you for the OWL file and you can upload the file from your desktop onto the Stanford Web Protege server. It's pretty secure since the URL that's generated is not something someone would stumble on so only people that you give the URL to can see it. I think you can even control if more by specifying only specific other Web Protege users that can see it.
The one issue with Web Protege is that as of now it doesn't support a reasoner. That's one reason I find the option: "File>Export inferred axioms as ontology" so useful. When I'm going to save an ontology into Web Protege I make sure that I first run the Reasoner and then use the Export inferred axioms option and upload the results of that export to Web Protege. That way users who view via Web Protege will see a file with all the inferences that the Reasoner has made. FYI, there is a minor bug in that option where it doesn't append ".owl" to the generated file an you need to manually go and add that to the file after it's saved (or include ".owl" in the name of your file when you do the export.