Answer: C, 5V DC.
Answer analysis: When discussing the driving voltage of LED display unit boards, we need to consider the characteristics of LED lamp beads and common driving methods. Now, let's analyze the given options one by one:
A, 40V DC: Although DC voltage is a possible way to drive LEDs, 40V DC voltage is too high for most LED display unit boards. LED lamp beads in LED displays are usually driven at low voltages (such as 3.3V or 5V), so this option is unlikely to be the correct answer.
B, 220V AC: AC voltage is commonly used for household and industrial power supply, but it is not suitable for directly driving LED display unit boards. LED displays require stable DC voltage to ensure the normal operation and long life of LED lamp beads. Therefore, this option can be excluded.
C, 5V DC: This is one of the most common driving voltages for LED display unit boards. Since LED lamp beads themselves have lower driving voltage requirements, using a 5V DC power supply can easily meet this demand. At the same time, a 5V DC power supply is also relatively stable and safe, suitable for long-term operation and parallel driving of a large number of LED lamp beads.
D. 5V AC: Although the voltage value is the same as option C, AC voltage is not suitable for directly driving the LED display unit board. As mentioned earlier, the LED display requires a stable DC voltage to ensure normal operation. Therefore, this option can be excluded.
In summary, the LED display unit board is generally driven by 5V DC. Therefore, the correct answer is C, 5V DC.