Answer: C. Plasma.
Analysis: When discussing this issue, we need to understand the relationship between each option and the digital display.
A. Nixie tube: Nixie tube is a commonly used digital display device that can display numbers and characters in two ways: dynamic display and static display. Therefore, the nixie tube is part of the digital display type.
B. Light-emitting diode (LED): Although LED itself is a light-emitting diode, it is also often used as the light-emitting component of digital tubes. In addition, LED display is also a type of digital display technology. So, light emitting diodes (LEDs) are related to digital displays.
C. Plasma: Plasma technology usually refers to plasma display screen (PDP), which is a display technology used in televisions and other display devices. It mainly uses the principle of gas discharge to produce images. But "plasma" here as a concept or technology is not specifically aimed at digital display. It has more to do with image display than just numbers.
D. Self-balancing display meter: Although this term may be less common, if we understand it as a digital display meter, then it is likely to be related to the digital display.
To sum up, what is not a digital display type is C. plasma. Because it is mainly an image display technology, not just a digital display.