The correct answer is option 4: Australia. Exercise Malabar is a naval exercise that is conducted annually between the United States of America, Japan, and India. It aims to enhance maritime cooperation and interoperability among the participating countries. Australia did not participate in the 21st edition of Exercise Malabar.

Option 1, the United States of America, is a participating country in Exercise Malabar. Option 2, Japan, is also a participating country. Option 3, India, is the host country and actively participates in the exercise.

Exercise Malabar is a significant event for strengthening maritime security and promoting cooperation between the Indian Ocean and the Pacific Ocean. The exercise involves joint training activities such as surface warfare, anti-submarine warfare, and maritime interdiction operations. It helps foster better understanding and coordination among the navies of the participating countries.