What is the MOST likely cause of an LCD screen that suddenly becomes dark while downloading a large file?

The most likely cause of an LCD screen becoming dark while downloading a large file is that the inverter in the LCD assembly has malfunctioned. The inverter is responsible for converting DC power from the laptop’s battery or power adapter to a high voltage AC power needed to drive the cold cathode fluorescent lamps (CCFLs) that illuminate the screen. When the inverter malfunctions, the screen may not get the necessary power, resulting in a dark display.

In the context of the question, while option A mentions that the cold cathode fluorescent lamp has burned out, if it were truly a burned-out lamp, the screen would likely remain dark regardless of the file download. The inverter could fail intermittently when the system is under load, such as during a large file download, leading to issues with screen brightness on an LCD.

The issues related to system's battery being low could result in performance throttling or the system shutting down, but it wouldn’t directly cause the display to go dark under normal operation. A faulty wireless network card also would not affect the display's brightness. Therefore, the inverter malfunction is the most plausible explanation for the sudden darkness of the LCD screen in this scenario.