When did Google announce its mobile-first indexing initiative and re-introduce the Mobile Friendly Testing Tool?

Google officially announced its mobile-first indexing initiative in November 2016. This was a significant shift, indicating that Google would primarily use the mobile version of a website's content for indexing and ranking, rather than the desktop version. At the same time, Google re-introduced and emphasized the Mobile-Friendly Testing Tool to help webmasters understand how their sites performed on mobile devices and prepare for this change. The other dates are incorrect; while mobile optimization has been a long-standing focus for Google, the specific announcement and tool re-introduction for mobile-first indexing occurred in November 2016.