Many Europeans (explored/expanded) the continent of Africa in 19th century.what shall I choise?
my teacher said i should chiose"explored" but I think I should chiose "expanded".what shall i chiose?

Question

Answer

choose. You should choose "explored".

"They explored the continent of Africa..."
They went to Africa and had a look. Made maps.

"They expanded the knowledge about the continent of Africa in the 19th century."
They told other people about Africa. Expanded their knowledge.

Answer

If you use "expanded the continent of Africa", then that means you have made Africa physically larger.  Or inflated.  A feat of engineering!

So be sure about what you're expanding - Domasla's suggestion of "expanded the knowledge" makes perfect sense.