While Columbus is generally cited as the European who "discovered" America, thinking that he was in East Asia, and it was Amerigo Vespucci correctly deducing that the New World was a landmass previously unknown to the Europeans.

Understanding- the underlined portion thinking that he was in East Asia falls inside two commas hence the sentence should be an independent clause (forming a complete meaningful sentence)

A) thinking that he was in East Asia, and it was Amerigo Vespucci correctly deducing
WRONG thinking that he was in East Asia is not an independent clause and discovered is not parallel with the word deducing

B) he thought that he was in East Asia, and it was Amerigo Vespucci who correctly deduced
CORRECT No parallelism error and consists of the required independent clause

C) he thought that he was in East Asia, instead of Amerigo Vespucci who correctly deduced
WRONG he thought that he was in East Asia is not an independent clause since it alone fails to make a meaningful sentence

D) thinking that he was in East Asia, but Amerigo Vespucci correctly deduced
WRONG Lacks independent clause

E) in East Asia according to his thought, and Amerigo Vespucci correctly deduced
WRONG Lacks independent clause