Which term would best describe a highly volatile printer?

The term "temperamental" is the best choice to describe a highly volatile printer. It implies that the printer is inclined to sudden mood changes or unpredictable behavior, much like a person who can be moody or capricious. This descriptor captures the essence of volatility, suggesting that the printer may not operate consistently and could exhibit erratic behavior in its performance.

In this context, the other terms do not effectively convey the intended meaning. "Equanimous" refers to someone who is calm and composed, which stands in stark contrast to the idea of being volatile. "Truculent" suggests a fierce or aggressive demeanor that does not align with the nature of a dysfunctional printer. Similarly, "petulant" implies a sulky or irritable temperament, typically associated with childish behavior, rather than the unpredictability characteristic of a volatile device. Therefore, "temperamental" is the most appropriate descriptor for a highly volatile printer.