a) section
b) header
c) nav
d) aside

Answer: c
Explanation: The nav element is for marking up the navigation links or other constructs (eg a search form) that will take you to different pages of the current site, or different areas of the current page. Header contains section of headings. Section element is used to make sections in document. The aside tag defines a block of content that is related to the main content around it, but not central to the flow of it.