Why the answer is C, and why the others tempt you.
Shirk means associating partners with Allah and is the direct opposite of Tawhid, which affirms Allah's absolute oneness. Kufr is disbelief in general, Nifaq is hypocrisy, and Bid'ah is religious innovation. While these are serious offences, Shirk is specifically the antithesis of Tawhid and is described in the Qur'an (Q4:48) as the one sin Allah does not forgive if one dies upon it.
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