Multiple expansion cards are installed in a rack mount 1U server. All expansion cards stop working simultaneously. Which of the following is the MOST likely cause of the problem?

While a motherboard failure could cause this, it is less specific than a riser card failure when specifically referring to *all expansion cards* in a rack server, as the riser is the common intermediary.

An operating system issue might cause cards to appear non-functional due to driver problems, but it is less likely to cause a simultaneous physical cessation of function for *all* cards at the hardware level.

It is highly improbable for multiple physically separate expansion cards to fail simultaneously on their own without a common point of failure.

In a 1U rack-mount server, expansion cards often do not connect directly to the motherboard due to space constraints; instead, they connect to one or more riser cards, which then connect to the motherboard's PCIe slots. If all expansion cards fail simultaneously, the most likely single point of failure affecting all of them is the riser card, which provides power and data connectivity to all attached expansion cards.