The largest lake in the world by both area and volume is (A) Caspian Sea. The Caspian Sea, although called a "sea," is technically the largest lake on Earth. It is located in Central Asia and is bordered by five countries: Russia, Kazakhstan, Turkmenistan, Iran, and Azerbaijan. The Caspian Sea has significant economic and environmental importance for the surrounding regions and plays a crucial role in the geopolitical landscape of the area.

The decayed remains of living organisms in soil are called (B) Humus. Humus is the organic material that forms as a result of the decomposition of plant and animal matter. It is a dark, nutrient-rich component of soil and plays a vital role in soil fertility. Humus improves soil structure, retains moisture, and provides essential nutrients for plant growth. It is a key element in sustaining healthy and productive soils in various ecosystems.

The sea that does not belong to Russia is (B) Baltic Sea. The Baltic Sea is located in Northern Europe and is bordered by several countries, including Sweden, Finland, Denmark, Germany, Poland, Latvia, Estonia, Lithuania, and Russia. It is not entirely enclosed by Russia. The other seas mentioned (Laptev Sea, Kara Sea, and White Sea) are all located in the Arctic and subarctic regions and are part of the Russian territory.

Downs grasslands, commonly known as "downs," are extensive grassy plains found in Australia. These grasslands are a distinctive feature of the Australian landscape, characterized by vast expanses of open grassy areas. They play a crucial role in supporting unique ecosystems and biodiversity in Australia. Downs are often associated with pastoral activities and are utilized for grazing livestock, especially in regions with a semi-arid to arid climate. These grasslands contribute to the ecological diversity of the Australian continent.

The Nile can be defined as the longest river by virtue of total head water lenght I.e from where the river is officially called the “Nile". In regard to discharge rate and catchment area the Amazon triumphs the Nile. The Amazon however has a much longer source stream than the Nile.

The Pacific Ocean is almost triangular in shape. It's not perfectly round, but it appears circular when viewed on maps and globes. The Pacific Ocean's apex is in the north at the Bering Strait. 

The world's longest river, the Nile, flows through the continent of Africa. It is approximately 6,650 kilometers (4,130 miles) long and is a significant geographical feature in northeastern Africa. Therefore, the correct answer is (B) Africa.