How were Chinese forces able to attack as MacArthur's forces marched northward?

Chinese forces were able to attack as MacArthur's forces advanced northward primarily by marching in between the UN forces. The strategic positioning allowed the Chinese People's Volunteer Army to effectively engage UN troops that were, at that time, under the command of General Douglas MacArthur. This maneuver exploited gaps in the UN defense, taking advantage of the dilapidation in communication and coordination among the allied forces.

The other options, while they may have elements of truth in isolated contexts, do not provide a comprehensive explanation of how the Chinese forces coordinated their attack during this specific operational phase. For instance, while amphibious tactics can be significant in military strategy, they were not the method employed by the Chinese in this situation. Similarly, the use of land mines along the Yalu River is not a primary factor contributing to their ability to mount a large-scale offensive against UN forces at this time. Thus, the actions of marching in between UN forces most clearly represent the actual strategy utilized by the Chinese forces during the conflict.