Step 1: Recall the three ways CSS can be applied to a page.
CSS can be written inline on an element, placed inside a style block in the document, or kept in a completely separate file and linked in, and this question is specifically about that third, external method.
Step 2: Learn the tag and attributes HTML actually uses for this.
To pull in an external stylesheet, HTML uses the link tag placed inside the head section, with rel=\"stylesheet\" telling the browser what kind of resource this is and href=\"styles.css\" pointing to where the file actually lives.
Step 3: Rule out the invented tags.
Tags such as css and stylesheet do not exist anywhere in HTML, and the link tag never uses style or src as attribute names for this purpose, so the syntactically correct line is the one using rel and href together.
\[ \boxed{\text{<link rel=\"stylesheet\" href=\"styles.css\">}} \]