I am going to say "C" for all. Muhammad was the prophet of Allah so I don't believe stating so is shirk. In fact in stating so you are just confirming that you are a follower of the message which he delivered (the Qur'an). Though the shahada is not mentioned in the Quran I don't see how it is forbidden by the Quran as there is nothing untrue in the statement. I do agree that some (not all) Quranist take things that Muslims do that are not in the Quran and advise against them because they are not mentioned in the Quran even though the Quran does not forbid it. While at the same time they criticize the Sunni and the Shia for banning things that are not mentioned in the Quran. (Again I would like to say some not all.)
I think you would be interested in reading this article http//americanunitarian.org/grantshahada.htm It's a Unitarian Christian advocating for a Unitarian Christian shahada. (The rest of the site is really informative too) As they believe in one God and Jesus as the messenger of God (and not God himself) I feel they are also submitters to God, not committing shirk and a part of our brethren.
I have also heard the argument brought forth about a Muslim being one who submits to the one God, regardless of whether they are a Jew, Christian or any other name they go by and a momin (believer) is one who believes in the message of the Quran brought by prophet Muhammad. The justification for this is linguistics based on the use of both words in the Quran. I have not done enough independent investigation to comment on this (maybe someone else here has), but there is a logic to this theory and a difference in shahada could distinguish this. (Though I am not saying it is necessarily required to. Just throwing around ideas.) For more on the Muslim/momin theory I suggest you read 'Notes on Islam' by Sir Ahmed Hussain. There is a free kindle download at Amazon http//www.amazon.com/Notes-on-Islam-ebook/dp/B0082X9Z76
Allah knows best.