Final answer:
The Ruffed grouse (B) is the species most closely associated with early successional forests due to its need for thick underbrush and open spaces that are typical in these habitats.
Step-by-step explanation:
The species most closely associated with early successional forests would be one that thrives in habitats characterized by recently disturbed areas with rapid plant growth and lower forest canopies. Among the given options, the Ruffed grouse (B) is known for its preference for early successional habitats, sometimes referred to as 'young' forests, as it requires thick underbrush for nesting and as protection against predators. The other listed species, such as the flying squirrel, the red-backed salamander, and the goshawk, have different habitat preferences that are not as closely tied to early successional stages.
An early successional forest is initially dominated by pioneer species and intermediate species like small pine, oak, and hickory trees before gradually developing into a climax community over many years. The presence of the ruffed grouse in these early stages suggests it is adapted to conditions such as thicker underbrush and open, sunlight-rich spaces created by such disturbances as forest fires.