China and India are currently the two most populous countries in the world. To determine which two countries follow China and India in terms of decreasing population, let`s evaluate each option:

Option 1: Brazil and USA

Brazil is the fifth most populous country in the world, with a population of approximately 214 million people as of 2021. The United States (USA) is the third most populous country, with a population of approximately 331 million people as of 2021. Therefore, Option 1 is not correct because the USA has a larger population than Brazil.

Option 2: USA and Indonesia

As mentioned earlier, the United States is the third most populous country. Indonesia, on the other hand, is the fourth most populous country in the world, with a population of over 276 million people as of 2021. Option 2 is correct because after China and India, the countries with the next largest populations are the United States and Indonesia.

Option 3: Canada and Malaysia

Canada is the 38th most populous country globally, with a population of approximately 38 million people as of 2021. Malaysia, on the other hand, is the 45th most populous country, with a population of over 32 million people as of 2021. Option 3 is not correct because neither Canada nor Malaysia are among the top five most populous countries in the world.

Option 4: Russia and Nigeria

Russia is the ninth most populous country in the world, with a population of approximately 144 million people as of 2021. Nigeria, however, is the seventh most populous country, with a population of over 211 million people as of 2021. Option 4 is not correct because Nigeria has a larger population than Russia.

In conclusion, Option 2: USA and Indonesia is the correct answer. The United States and Indonesia are the two countries that follow China and India in the decreasing order of their populations.