Which of the following items is part of an espresso machine?

An espresso machine is designed to brew coffee by forcing hot water through finely-ground coffee under pressure, and it typically includes a variety of components to facilitate this process.

A warming tray is often incorporated into espresso machines to keep cups warm before serving, ensuring that the coffee stays hot longer when it is poured. This is particularly important in maintaining the quality and temperature of espresso.

The drip tray is another essential component that collects excess water and coffee that may drip from the portafilter after pulling a shot or when steaming milk. This helps in keeping the work area clean and prevents spills.

The pressure gauge is crucial for monitoring the pressure during the espresso extraction process. Ideal brewing pressure for espresso is typically around 9 bars, and the pressure gauge helps the barista ensure that the machine is operating within this optimal range to produce a quality shot of espresso.

Since all these items contribute to the functionality and performance of an espresso machine, the correct answer encompasses all of them, making "All of the above" the right choice. Each component plays a significant role in ensuring that the espresso machine operates efficiently, delivers great-tasting coffee, and maintains cleanliness and readiness during service.