What does 'A tempo' indicate in a musical score?

The term 'A tempo' is an instruction in musical notation that signals to musicians to return to the original tempo after having altered the speed of the music, either by slowing down or speeding up. This directive typically comes after a section of the piece where the tempo may have changed, indicating that it is time to resume the piece's initial tempo. This term ensures that the piece maintains its intended rhythmic character throughout its performance, providing coherence in the musical flow.

In this context, the other options describe different tempo alterations but do not align with the meaning of 'A tempo,' which specifically refers to the return to the original speed, thus reinforcing the idea that the original tempo is being reinstated.