Explanation:

Recently, three underwater geographical structures located in the Indian Ocean for which the International Hydrographic Organization (IHO) and UNESCO’s Intergovernmental Oceanographic Commission (IOC) have awarded names originally proposed by India.

• It promotes international cooperation in marine sciences to improve management of the ocean, coasts and marine resources.

• The IOC enables its 150 Member States, and India has been a member of this commission since 1946.

• It works together by coordinating programmes in capacity development, ocean observations and services, ocean science, tsunami warning and ocean literacy.

• The IOC is in charge of coordinating the United Nations Decade of Ocean Science for Sustainable Development 2021-2030, the “Ocean Decade”.

Previously named structures are

• Raman ridge (accepted in 1992): It was discovered in 1951 by a US oil vessel. It was named after Physicist and Nobel Laureate Sir CV Raman.

• Panikkar Seamount (accepted in 1993): It was discovered by India research vessel Sagar Kanya. It is named after NK Panikkar, a renowned oceanographer.

• Sagar Kanya ridge (accepted in 1991): A seamount was named after the research vessel itself.

• DN Wadia Guyot: It was named after a geologist DN Wadia when an underwater volcanic mountain, known as guyot, was discovered in 1992 by Sagar Kanya.

• Recently two structures were named after the rulers of the Mauryan dynasty namely Ashoka seamount and the Chandragupt ridge and one more was named as Kalpataru ridge in the Indian Ocean.

Hence option 4th is correct.