Which CSS selector targets an element by its tag name?

The correct option is p { ... }.

• .left-nav { ... } is a class selector, targeting elements with the class left-nav.
• .id { ... } is also a class selector, targeting elements with the class id.
• #footer { ... } is an ID selector, targeting the element with the ID footer.
• p { ... } is a tag selector (or element selector), targeting all HTML <p> (paragraph) elements.

The selector that styles an element by targeting its tag name is p { ... }.

$\boxed{\text{p \{ ... \}}}$