Assigning a new IP address range to the local company is the best option to quickly connect the two companies without causing any IP address conflicts or overlaps. This option requires reconfiguring the local company's network devices and updating the routing tables on both sides, but it avoids the need for any NAT or static routing solutions that may introduce additional complexity, cost, or performance issues12 References1: Connecting Networks with Overlapping IP Ranges 2: What Is Network Address Translation (NAT)?

The structure of the management data of a device subsystem using a hierarchical namespace containing object identifiers

Explanation
To identify the source and destination addresses of the traffic causing network saturation, the network administrator should use a network protocol analyzer that supports the NetFlow protocol. NetFlow is a network protocol that collects IP traffic information as it enters or exits an interface and sends it to a NetFlow collector for analysis. This data includes the source and destination addresses of the traffic, the ports used, and the number of bytes and packets transferred.
Therefore, the correct answer is option D, NetFlow.
Reference: CompTIA Network+ Study Guide, Exam N10-007, Fourth Edition, by Todd Lammle (Chapter 6:
Network Devices)

Port security is a feature that allows network administrators to limit the number of devices that can connect to a switch port, based on the MAC address of the device. This can prevent unauthorized physical connectivity by blocking any device that is not on the allowed list or exceeding the maximum number of devices per port.
Port security can also trigger an action, such as shutting down the port or sending an alert, when a violation occurs. References: CompTIA Network+ N10-008 Cert Guide - O'Reilly Media, Chapter 14: Securing a Basic Network, page 512