Final answer:
The command to enable an interface on a router to acquire an IPv4 address automatically from an ISP is 'ip address dhcp'. This command utilizes DHCP to obtain an IP address from the ISP.
Step-by-step explanation:
The correct command to enable an interface on a router to acquire an IPv4 address automatically from an ISP is ip address dhcp. This command configures the interface to use DHCP (Dynamic Host Configuration Protocol), which allows it to request and obtain an IP address from a DHCP server provided by the ISP.
When the link to the ISP is enabled, the router will send a DHCP discover message, asking for an IP address. The DHCP server will then assign an available IP address to the router's interface, allowing it to have network connectivity.
Using DHCP simplifies the network configuration process, as it eliminates the need to manually assign IP addresses. It also allows for dynamic IP allocation, which means that different devices can receive different IP addresses each time they connect to the network.