Im Wesentlichen in dem du keine Displaymath-Umgebung benutzt.
Code:
\documentclass[a4paper,10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage[french,english,ngerman]{babel}
\begin{document}
\begin{enumerate}
\item
$%\displaystyle %ggfs
\begin{aligned}[t]
\left(M_t\right)^2-E\left[\left(M_t\right)^2\right],\quad Var\left[X_t\right] < \infty,\forall t \geq 0
\end{aligned}
\hfill\refstepcounter{equation}(\theequation)
$
\item
$%\displaystyle %ggfs
\left(M_t\right)^2-E\left[\left(M_t\right)^2\right],\quad Var\left[X_t\right] < \infty,\forall t \geq 0
\hfill\refstepcounter{equation}(\theequation)
$
\end{enumerate}
\end{document}
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