The first page of a website is called the

The home page is the main entry page that opens first when a website is visited.

An index can list contents but is not the standard name for a site's opening page.

A menu bar is a strip of navigation links, only a part of a page, not the page itself.

No other choice fits because the home page is exactly this first page.

The home page usually carries the main menu and links into the rest of the site.

A home page is often reached through an address ending in index.html or the bare domain.