Step 1: Understanding the Question:
The question asks for one specific calendar date that both Bangladesh and India observe every year to mark a major event from 1971, though under two different names.
Step 2: Key Fact or Approach:
The Bangladesh Liberation War of 1971 ended with a formal military surrender in Dhaka, and that surrender date became the anniversary both countries now observe. Matching the historical date to the calendar options given is enough to answer this.
Step 3: Detailed Explanation:
On 16 December 1971, the Pakistani Eastern Command surrendered to the joint command of the Indian Army and the Mukti Bahini in Dhaka, ending nine months of war and creating the new nation of Bangladesh.
Bangladesh calls this date Bijoy Dibosh, its national Victory Day, celebrated with parades and state ceremonies each year.
India calls the same date Vijay Diwas, honouring the soldiers of the 1971 war with wreath laying ceremonies at military memorials.
The other three dates, 15 December, 21 December and 28 November, do not correspond to this surrender or to any matching dual observance by both countries.
Step 4: Final Answer:
16 December is the date jointly marked as Victory Day in Bangladesh and Vijay Diwas in India, so option (b) is correct.