what is the most ideal growing conditions for citris fruits such as oranges?
When growing citrus trees it is ideal to have average daytime temperatures of 25C and plenty of direct sunlight, irrigation will probably also be required.
where are most of the world's oranges grown?
Brazil is the nation that produces the largest crop of Oranges in the world, accounting for over 21% of the world's orange production by weight. However, China and India together produce more oranges than Brazil does alone, so looking beyond national borders there is an argument to be made that South Asia is the world's largest orange producer, with just over 25% of the world's production.

That said, there is no one place where most of the world's oranges are grown. Oranges and other citrus fruits are grown all around the world, wherever temperatures are warm, sun is plentiful, and water available.