Try like this

\begin{enumerate}

\item $\varnothing, X\in\mathcal{T}$

\item $\forall s \in\mathcal{T}, \displaystyle{\bigcup_{A\in S}} A \in\mathcal{T}$

\item $\forall s \in\mathcal{T}, s\ \mbox{finite}, \displaystyle{\bigcap_{A\in S}} A \in\mathcal{T}$

\end{enumerate}