I've seen adagio, and other names, but I see in classical score that indicates "Tempo 1" in section C.
What does it mean?
Tempo 1 or Tempo I instructs a performer to return to the first tempo of a movement or piece of music, where there has been a different tempo marking since the first marking. The marking Tempo Primo is also used.
It is the equivalent, on a larger scale, of an a tempo marking following a rit. or rall. marking.
Tempo primo, or Tempo 1ᵒ means “at the same tempo as the piece started”.
Without seeing a picture, I would guess that section A had some tempo, section B some other and section C has to return to the first tempo, to the tempo of section A; thus 'Tempo 1'