what is the structure of the following sentence?
although i have already visited africa, i would certainly like to return.
a. compound-complex
b. compound
c. simple
d. complex

The structure of the sentence Although I have already visited Africa, I would certainly like to return is D. Complex.

Explanation:

A complex sentence is the one that has one independent clause -one that can stand alone- and one dependent clause. In this case, the independent clause is I would certainly like to return and the dependent clause is although I have already visited Africa.