The best answer is option C. This option is the one that includes the information given in the original sentence, but summarized (avoiding giving extra information about what marine algae are, or using complicated grammatical structures.)
Option D is quite short, but summarizes the main idea of the original sentence without giving any extra information at all.
None of these two options (C and D) change the overall meaning of the original sentence, but one of them (C) gives the reader more information about the points mentioned in the original sentence ("<em>more than all of the earth plants and trees combined</em>"), different from the other option (D) which goes straight to the point thorugh the use of a simple sentence.
