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.

(A) Species that are found only in some particular areas

Endemic species are those that are native and restricted to specific geographical areas and are not naturally found anywhere else in the world. They have evolved and adapted to particular local conditions, making them unique to those specific regions.

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 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.

The steppe grassland is primarily found in (A) Asia. The Eurasian Steppe, also known as the Great Steppe, is a vast expanse of grassland that stretches from Eastern Europe to Siberia in Asia. It is one of the world's largest temperate grasslands and spans several countries, including Russia, Ukraine, Kazakhstan, and Mongolia.

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 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 land of white elephants is (C) Thailand. This term is often used metaphorically to refer to Thailand, highlighting its cultural richness and uniqueness. In Thai culture, the white elephant is considered a symbol of good luck and prosperity.