Final answer:
Option (A), Poush 1st is seen as a dark day due to political instability that resulted in reduced tourism and economic depression in Nepal.
Step-by-step explanation:
Poush 1st is regarded as a dark day in the history of Nepal largely due to political instability. This period was characterized by frequent protests, civil unrest, and conflicts between those loyal to the royal family and those seeking a republic, which discouraged tourism and further depressed the economy.
The struggle for a stable government, including the Maoist Communist movement and other political disruptions, culminated in a peace accord and led to the establishment of a federal democratic republic with the first president of Nepal sworn into office in 2008. Despite these changes, the tension and challenges to economic and political progress remained significant.