The largest ecosystem on the planet is the ocean system. The correct option in regards to the given question is option "C". It is a known fact that our earth is covered with 70% water and 30% land. On top of that the marine environment covers about 97%. This is the largest ecosystem on our planet. It is a known fact that our earth is covered with 70% water and 30% land. On top of that the marine environment covers about 97%. This is the largest ecosystem on our planet. The marine world supports a huge amount of life form and there is also the opinion that the first life could have evolved from this system.