The correct answer is option 'D', which states that over 90% of the world's biomass is in the oceans. Let's explore why this is the case.

1. Definition of Biomass:
Biomass refers to the total mass of all living organisms in a given area or ecosystem. It includes plants, animals, fungi, and microorganisms.

2. Importance of Oceans:
The world's oceans cover about 71% of the Earth's surface and play a crucial role in sustaining life on our planet. They are home to a wide variety of plants and animals, ranging from microscopic phytoplankton to large marine mammals.

3. Phytoplankton:
Phytoplankton are microscopic, photosynthetic organisms that form the base of the marine food chain. They are responsible for producing a significant portion of the Earth's oxygen and are a major source of food for many marine organisms. Phytoplankton biomass is exceptionally high in the oceans, contributing significantly to the overall biomass.

4. Marine Food Chain:
The oceans support a complex food chain, where organisms feed on each other. Small fish feed on phytoplankton, and larger fish feed on those small fish. This process continues up the food chain, with larger predators feeding on smaller ones. As each level of the food chain accumulates biomass, the overall biomass in the oceans increases.

5. Biodiversity in Oceans:
The oceans are incredibly diverse, with a wide range of habitats, from shallow coral reefs to deep-sea trenches. This diversity supports a vast array of species, each contributing to the overall biomass. Marine organisms such as fish, whales, dolphins, sharks, and countless other species collectively make up a significant portion of the world's biomass.

6. Global Carbon Cycle:
The oceans also play a crucial role in the global carbon cycle. They absorb a substantial amount of carbon dioxide from the atmosphere, helping to regulate the Earth's climate. This carbon is incorporated into the biomass of marine organisms, further contributing to the overall biomass in the oceans.

In conclusion, the vastness of the oceans, coupled with their high productivity, diverse habitats, and important roles in the global ecosystem, makes them the primary contributor to the world's biomass. Over 90% of the Earth's biomass can be found in the oceans, highlighting their significance in supporting life on our planet.