Tikzcd commutative diagram

Question

I have been trying to draw the following diagram (see attached photo) but just can’t seem to get the syntax right. I have been looking at a tikzcd tutorial off the net but clearly I haven’t quite figured it out. Any suggestions gratefully received.

Answer 1

With tikz-cd:

\documentclass{book}
\usepackage{tikz-cd}
\begin{document}

   \begin{tikzcd}
   & C\\
   A\ar[r]\ar[d]\ar[ur] & B\ar[u]\\
   D\ar[ur]\ar[uur, bend left=75]
   \end{tikzcd}
\end{document}