Explanation:

The Indian Ocean Naval Symposium (IONS) is a voluntary initiative that seeks to increase maritime cooperation among navies of the littoral states of the Indian Ocean Region.

• The 25 member states of the IONS are classified under four sub-regions, namely the South Asian sub-region, the West Asian sub-region, the South-East Asian and Australian sub-region, and the East African sub-region.

The 25 member nations of the IONS are:

• South Asian sub-region: India, Pakistan, Bangladesh, Maldives, Sri Lanka
• West Asian sub-region: Bahrain, Oman, United Arab Emirates, Iran.
• South-East Asian and Australian sub-region: Australia, Indonesia, Malaysia, Singapore, Thailand.
• East African sub-region: Kenya, Tanzania, Mauritius, Seychelles, Hence, option D is correct.
• Additionally, there are eight states with observer status in the IONS: China, Germany, Italy, Japan, Madagascar, Netherlands, and Russia.

Hence option 4th is correct.