Rufus developer here. @kirkru is 100% right, there is absolutely no point in selecting FreeDOS or changing the Partition scheme or Target system options because these will be reset as soon as you select the Ubuntu ISO.
Also it may be worth mentioning that Rufus can compute the MD5/SHA1/SHA256 of the ISO if you click the blue (✓) button after selecting the image.
Finally recent versions of Rufus have added persistence support, so you will see an extra set of fields allowing you to set the size of the persistent partition (with a default of 0
meaning no persistence).