An engineer is building a web page and wants to add some styling to their content. They decide to store their CSS styling in a separate file named app$-$styles.css$,$ which they will reference in their HTML via a link element.

This app$-$styles.css file is known as

a$.)$ an internal style sheet

b$.)$ third$-$party styling

c$.)$ an external style sheet

d$.)$ inline styling