Below is a question I was solving in an online exam for practice
And below they have given the solution. My question is the relation $(P(S),\subseteq)$ is itself not a chain because say neither $\{1\}$ $\not\subseteq \{2\}$ nor $\{2\}$ $\not\subseteq \{1\}$. So, how come they have given the size of the maximum chain to be 51?