Explanation:

Konkan Shakti 2021 was a landmark event as it marked the first-ever tri-services joint exercise between the armed forces of India and the United Kingdom (Option C). This exercise aimed to enhance interoperability and strengthen defense cooperation between the two nations across land, sea, and air domains. India conducts various military exercises with Israel (A), Mongolia (B), and Russia (D), but Konkan Shakti 2021 was specifically with the UK.