Explanation:

During India's Republic Day parade in 2021, a special tri-service contingent from a neighboring country participated to commemorate important bilateral ties:

- Option 4: Bangladesh - A tri-service contingent from Bangladesh participated in the 2021 Republic Day parade in India. This was significant as 2021 marked the 50th anniversary of Bangladesh's independence, highlighting the strong historical and diplomatic ties between the two nations. Correct Answer

- Option 1: Myanmar - Did not have a contingent participate in the 2021 parade.

- Option 2: Nepal - Did not participate with a tri-service contingent in the 2021 parade.

- Option 3: Bhutan - Also did not have a contingent present at the parade in 2021.