Final answer:
Wi-Fi is the most likely network adapter to be preinstalled in a typical notebook computer today, as it is essential for wireless connectivity and mobile use.
Step-by-step explanation:
The network adapter that is most likely to be preinstalled in a typical notebook computer today is Wi-Fi. Wi-Fi adapters allow notebooks to connect to wireless networks, which is crucial for portability and convenience. Most notebooks are designed to be mobile and therefore come equipped with Wi-Fi capability. Other common preinstalled adapters might include Bluetooth for connecting peripherals like a mouse or headphones, and some notebooks still include an Ethernet port for wired connections, but Wi-Fi is virtually standard. A USB is not a network adapter, but rather an interface for connecting various peripherals.