Which of the following is not a valid way to overlload the $*$ operator in $C++?$

int operator$+($const int &$.$ lhs$,$ const int& rits$)[$returm llhs $+$ rhs: $]$

void operatort $($const int & : lls$,$ const std:string $E$ rits$)[$ return lhs $+$ rhs: $]$

std:$-$string operator$+($const int $E$ l lhs$,$ const char" riss$)\{$ return llhs $+$ rhs: $]$

std$-$string operator$+$f$($const std string & llhs$,$ const std$-$string& rhs$)[$return lhs $+$ rhs: $]$