When every heading in a document changes together, they are all governed by a shared paragraph style, so editing that style updates all headings using it — which is not what the user wants here.

To change only the one selected heading, the user should apply Direct Formatting: select just that heading and change its font size manually, which overrides the style for that text alone.

Direct (manual) formatting sits on top of the underlying style and applies only to the specific selected text, leaving all the other headings untouched.

Using Format Painter would copy formatting from one place to another rather than isolate a single heading's change, so it does not solve the problem.

Modify the Style is the opposite of what is needed: it edits the style definition and would therefore change every heading using that style.

Changing the Theme alters the fonts and colours across the entire document, again affecting all headings rather than just one.

Because only manual overriding of the selected heading achieves the goal, the correct answer is Apply Direct Formatting.