While submitting a new app to the App Store I got this question: "Does your app contain, display, or access third-party content?".
Can I answer yes to this question and still submit an app?
Does answering yes mean that I have to show confirmations from the creators of the third-party content that I can use it in my app?
If for example my app is Twitter - would I have to answer "yes" in this case? If so, how could I prove that I have the rights for all content published in the app?
You seem to be of the understanding that answering "yes" to the question means that your app is not allowed on the App Store - that is not the case. You can just answer "yes" and continue with your app submission.
You will be asked if you have the rights to use the contents, or otherwise follow relevant laws in the countries where you will sell your app. If you cannot answer yes to that, you have a problem - not so much with Apple, but rather by risking getting sued (or worse).
If you do indeed answer yes, you do not need to provide any confirmations or documentation at that moment. You'll be able to continue app submission right away.
However note that as everyone else you have to follow the rules of the App Store Guidelines set forth by Apple. In particular, Apple requires that you upon request can document your authorization to use third party material, such as trademarks or patents. I guess this is likely to happen if a third-party complains to Apple that you're not authorized.
