Why A truth table is the complete input/output map of a digital logic device: every possible combination of input logic levels is listed on the left, with the resulting output state on the right. For a two-input gate that means 4 rows (2^2), for three inputs 8 rows (2^n rows for n inputs). It fully defines the gate's behavior without needing a schematic or equations.
Watch out The op-amp choices are out of place here, since op-amps are analog devices with continuously variable outputs, not discrete logic states. The choice limiting the table to only true outputs is wrong because a truth table lists both the low and high output cases.
Truth table = ALL input combinations and their outputs, digital only. 2^n rows for n inputs.