This is an explanation question. As usual in this type of question, there is an apparent paradox. In the second sentence, it appears that deforestation contributes to huge amount of greenhouse gas emissions--half of Santilia's total. The next sentence suggests that Santilia's total greenhouse gas emissions is a small number--less than 1% of the global total. The correct answer will explain how greenhouse gas emissions are a concern in Santilia despite representing such a small amount of the global total:

(A) This may be true, but it doesn't explain the large percentage of emissions stemming from deforestation.
(B) This choice doesn't address deforestation, nor does it have a clear connection to Santilia's total greenhouse gas emissions (industrial pollutants may fall into that category, but GMAT questions are usually more straightforward when you are expected to make a connection like that).
(C) This explains a possible remedy for deforestation, but it doesn't resolve the difference in claims regarding the country's emissions.
(D) This is correct. Since Santilia is a small country, it can have a serious emissions problem but still represent a small amount of the world's total. That the country was once covered with rainforest also explains how deforestation makes up such a large portion of the total.
(E) The influence of activists has little to do with the accuracy of their claims, which is the concern of this question.