Which of the following phrases signifies an instruction for dynamics in music?

The phrase "forte" signifies an instruction for dynamics in music, indicating that the passage should be played loudly. Dynamics in music refer to the volume of sound, and terms like "forte" are essential for conveying how intensely a section should be performed.

Other options, while relevant to musical expression, convey different aspects. "Con spirito" denotes playing with spirit or brightness, focusing on the character of the performance rather than its volume. "Con espressione" similarly emphasizes expressive playing, guiding musicians to interpret the piece with emotional depth rather than specific loudness. "Staccato" indicates how notes should be articulated—specifically, played in a detached manner—rather than focusing on their dynamic level. Thus, "forte" is the clear choice for instruction on dynamics.