The correct answer is: colossal.
The word “colossal” means “extremely large or impressive”. In this context, it is used to describe the speaker’s surprise at the man’s lack of knowledge about Africa. The man’s lack of knowledge is so great that it is almost unbelievable.
The other options are not as appropriate. The word “abundant” means “existing in large quantities”. This does not accurately describe the man’s lack of knowledge, which is not simply a matter of there being a lot of information about Africa that he does not know. The word “huge” means “very large”. This is a more accurate description of the man’s lack of knowledge, but it does not convey the speaker’s surprise as well as the word “colossal” does. The word “great” means “very good or impressive”. This is not an accurate description of the man’s lack of knowledge, which is not good or impressive.