\documentclass[12pt]{article}
\usepackage{amsmath, amssymb, graphicx, array, fancyhdr, unicode-math, multirow}

\DeclareMathOperator{\Var}{Var}
\DeclareMathOperator{\sd}{SD}
\DeclareMathOperator{\Pois}{Pois}
\DeclareMathOperator{\Bin}{Bin}
\DeclareMathOperator{\NegBin}{NegBin}
\DeclareMathOperator{\sech}{sech}


% Text: Calibri, all bold
\setmainfont{Calibri}
\renewcommand{\seriesdefault}{b}

% Math: choose a math font and visually embolden it
\setmathfont{TeX Gyre Termes Math}                  % base math font
\setmathfont[FakeBold=1.9]{TeX Gyre Termes Math}    % embolden; adjust 1.6–2.4

% set header
\pagestyle{fancy}
\fancyhf{} % clear defaults
\lhead{\resizebox{!}{2cm}{D}}
\chead{\raisebox{0.5cm}{\Huge Introduction}}
\rhead{\resizebox{!}{2cm}{D}}
\setlength{\headheight}{2.5cm} % slightly larger than your 2cm image

% start document
\begin{document}
\thispagestyle{fancy} 

\begin{table}[h!]
\centering
\renewcommand{\arraystretch}{1.2} % a little vertical padding
\begin{tabular}{m{1.2in} m{4in}}
% -------------------- Single --------------
\multirow{3}{*}{\centering\fbox{\includegraphics[width=1in]{images/single.png}}} 
  & $A = \{1, 2, 3\}$ \\
  & $S = \{4\}$ \\ 
  & $T = \{1\}$ \\ 
\\
\\
% -------------------- Abstract --------------
\multirow{2}{*}{\centering\fbox{\includegraphics[width=1in]{images/abstract.png}}} 
  & $S \subset A \Rightarrow \emptyset $\\
  & $T \subset A \Rightarrow \{1\} $\\
\\
\\
% -------------------- Question --------------
\multirow{3}{*}{\centering\fbox{\includegraphics[width=1in]{images/question.png}}} 
  & $A = \{1, 2, 3, 4, 5, 6\}$ \\
  & $S = \{7\}$ \\
 \\ & $S \subset A \Rightarrow ?$ \\
\end{tabular}
\end{table}

% second page
\newpage
\thispagestyle{fancy}

\begin{table}[h!]
\begin{tabular}{m{1.2in} m{4in}}
% -------------------- Answer --------------
\multirow{3}{*}{\centering\fbox{\includegraphics[width=1in]{images/key.png}}} 
& $A = \{1, 2, 3, 4, 5, 6\}$ \\
& $S = \{7 \}$\\ 
\\& $S \subset A \Rightarrow \emptyset$
% -------------------- Answer --------------
\end{tabular}
\end{table}

\end{document}