Which of the following marks a gradual increase in volume?

The term that indicates a gradual increase in volume is rinforzando. This dynamic marking is often used to direct performers to gradually build intensity, typically in a passage of music that requires more emphasis or climactic moments. It creates a swell in sound that can enhance the emotional impact of the music.

In contrast, forzando refers to a sudden emphasis on a single note or chord rather than a gradual increase. Sforzando, while similar, also denotes a strong, sudden accent but does so on specific notes, not as a continuous increase in volume. Con brio translates to "with vigor" and describes the character or energy of the music rather than how the dynamics should evolve. Thus, rinforzando is the best choice for representing a gradual swell in volume.