Which of the following means returning to the previous tempo?

The term that signifies returning to the previous tempo is "At tempo." This instruction is commonly used in musical scores to indicate that the performer should revert to the earlier established tempo after a section that may have involved a change, such as a slowdown or acceleration.

In contrast, "Accelerando" refers to gradually increasing the tempo, while "Affettuoso" is an expressive term indicating a tender or affectionate manner of playing, which does not relate to tempo changes. "Allargando" means to broaden, typically referring to a gradual slowing down while also expanding the dynamics or expression of the music. Thus, "At tempo" is the most accurate term that conveys the idea of returning to the tempo previously set in the piece.