The form options are to set what follow up page you want the lead to be directed to upon completion of the form.
As Mark said, the flow for sending the email will look something like this:
I keep my assets for white papers, ebooks, guides, all in one program. That includes the landing page, follow up page, email confirmation, social promotion, and smart campaigns. That way I can easily track all the leads that downloaded that asset and which channel they came from.
You can set the landing page to "stay on page", or simply create a landing page that says something like "Thank you. We've emailed you the Whitepaper."
Then create a smart campaign that uses the "trigger" fills out form. There you will be able to also select the flow, "send email". As long as your email is already approved in Marketo, you can select it from the drop-down menu.
PS it's very important you use the trigger (lightning bolt) "fills out form", not the filter "filled out form". The trigger will be automatic every time someone completes the form.
Hope this helps!