Which letter is not homeomorphic to the letter $C$? Which letter is not homeomorphic to the letter $C$?
I think letter $O$ and $o$ are not homeomorphic to the letter $C$. Is that correct?
Is there any other letter?

That's right. Also, $B$ is not homeomorphic to $C$ since homeomorphic letters have same fundamental groups but $\pi_1(C) = 0$ and $\pi_1(B) = \mathbb Z \ast \mathbb Z$.
Alternatively, you could argue that if you remove any point from $C$, except one of the two endpoints, then $C$ is no longer connected whereas if you remove one point from $B$, no matter which point you remove, $B$ will still be a connected space. 
