The statement is false: consider the set $C=[0,1]\cup\\{2\\}$ in the space $\Bbb R$ with the usual topology. The interior of $C$ is $(0,1)$, which is not dense in $C$.
The statement is false: consider the set $C=[0,1]\cup\\{2\\}$ in the space $\Bbb R$ with the usual topology. The interior of $C$ is $(0,1)$, which is not dense in $C$.