= Solution
The four principles are:
* <Axiom of choice>: for every family $(A_i)_{i\in I}$ of nonempty sets, there exists a <function> $c$ on $I$ with $c(i)\in A_i$ for all $i$.
* <Cardinal comparability principle>: for any sets $A,B$, there is an <injection> $A\hookrightarrow B$ or an <injection> $B\hookrightarrow A$.
* <Well-ordering theorem>: every set admits a <well-order>.
* <Zorn's lemma>: every nonempty <partially ordered set> in which every chain has an upper bound has a maximal element.
In the last assertion the upper bound belongs to the partially ordered set; it need not belong to the chain.
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