Which interrogative pronoun should replace the blank space in the following sentence?: ___ happened?

Mar 7, 2018

color(red)("What")" happened?"

Explanation:

Of the interrogative pronouns:
$\textcolor{w h i t e}{\text{XXX}}$Who
$\textcolor{w h i t e}{\text{XXX}}$What
$\textcolor{w h i t e}{\text{XXX}}$Why
$\textcolor{w h i t e}{\text{XXX}}$When
$\textcolor{w h i t e}{\text{XXX}}$Where
$\textcolor{w h i t e}{\text{XXX}}$Which
$\textcolor{w h i t e}{\text{XXX}}$How
only What can be used without any additional word before "happened"