Linking an external stylesheet is a job for the $\texttt{<link>}$ element placed in the head. It needs two attributes: $\texttt{rel="stylesheet"}$ to declare the relationship and $\texttt{href}$ to point at the file.
That gives $\texttt{<link rel="stylesheet" href="styles.css">}$. The other choices fail because $\texttt{<css>}$ and $\texttt{<stylesheet>}$ are not real tags, and $\texttt{<style>}$ is for embedding rules directly, not for referencing a file with $\texttt{src}$.
\[\boxed{\texttt{<link rel="stylesheet" href="styles.css">}}\]