What does "preserving the logic expression" mean?
The wording is strange, but: all of these circuits implement a function \$F = f\left(A, B, C, D, E\right)\$.  If two circuits implement the same function \$f\$, then they are &quot;preserving the logic expression&quot;.
When you build a truth table for a logic function, that truth table is enumerating the output of the function for each given input.  If you enumerate all possible inputs (i.e., for the five-input function here, if you enumerate all 32 possible combinations of the five boolean input variables), then your truth table fully defines the function.
So equal truth tables means equivalent functions.