Is Python good for mobile development?

B >> the Tab is not included.. that's correct!

C

Groups Appears for organizations that have Chatter enabled. If you don’t add this item to the navigation menu, groups are automatically included in the set of Smart Search Items instead and the Groups item is available from the Recent section.

https://developer.salesforce.com/docs/atlas.en-us.salesforce1.meta/salesforce1/customize_s1_nav_about.htm