Final answer:
The decline in literary production by Northeast writers likely occurred in the 1880s, which aligns with the period of significant industrial and social transformations in America that would have influenced literary themes and production.
Step-by-step explanation:
When considering the decline in literary production by writers from the Northeast, it is important to look at the broader context of American industrialization and social change towards the end of the 19th century. The rise of industrialization, the shift from a rural to an urban society, and the economic turmoil characterized by periodic depressions all played a role in shaping the literary output. With the rise of the Realist movement in American literature, there was a turn towards subjects that reflected the immediate, contemporary society which could correlate with a modification in production in the Northeast. The labor-related upheavals and changes in economic conditions created new thematic concerns for writers, who now sought to address the pressing social issues of the day.
The period that corresponds to a noticeable change or reduction in literary material produced by Northeast writers is not explicitly stated in the provided information, but it can be inferred that this would align with the significant industrial and social transformations that American society was undergoing by the end of the 19th century and beginning of the 20th century. This suggests option (c) 1880s as the most likely time when Northeast writers began producing less material, as this is when America was experiencing profound changes due to industrialization and the tumultuous economic landscape that affected both consumption and leisure activities.