What term would indicate that a passage should be performed with a bold character?

The term that appropriately indicates that a passage should be performed with a bold character is 'sforzando.' This term comes from the Italian word for 'forced' and is used in musical notation to indicate that a note or chord should be played with strong emphasis or sudden force. This creates a dynamic contrast and adds drama to the performance, which aligns well with the idea of performing with boldness.

The other options do not convey the same sense of emphasis or boldness. 'Sostenuto,' for example, refers to a more sustained and often more lyrical style of playing. 'Spiritoso' indicates a lively or spirited manner but does not specifically imply boldness in the same way sforzando does. 'Sonore' suggests that a passage should be played with a rich, full sound, but this is more about the quality of tone rather than the forcefulness of the attack. Therefore, the most fitting term for expressing a bold character in music is sforzando.