Writing a negative number in text mode: $-$9 or $-9$?

$-9$

Minus nine is a number, so you should type the whole number in a single mode, in this case the math mode. If you can find a minus sign in text font, typing the sign and nine both in text mode is acceptable depend on the context.

Option C

\documentclass{article}

\usepackage{textcomp}

\begin{document}
\textminus 9
\end{document}

This produces a legit minus sign.

Although it's a bit overkill, you could use the \num macro of the siunitx package:

\documentclass{article}

\usepackage{siunitx}

\begin{document}

\Huge

In mathmode: \(-9\)

With \textsf{siunitx}: \num{-9}

\end{document}