Which of the following gives a reversible operation?

A logic gate gives the reversible operation if we can recover input data from the output.

For example, a NOT gate gives the reversible operation because we always know that the input is the reverse of the output.

For the given logic gate, let the input is $X$.

So, the output, $Y = \overset{¯}{X + X} = \overset{¯}{X}$ (NOT gate)