start with what the sentence sentence is trying to mean. Its meaning that Over 6M hectares of emerald bamboo groves flourish in China. The bamboo groves are one fifth of the world’s reserves. Also they spread from the saturated delta of the Yangtze to the borders of the Nanling mountains in the southern part of the country.
Now look for errors in the sentence
A. Over 6M hectares of emerald bamboo groves – one fifth of the world’s reserves – flourish in China, spreading from the saturated delta of the Yangtze to the borders of the Nanling mountains in the southern part of - Spreading here could modify either china or the groves. Even if we consider it to modify the entire earlier clause it would still have meaning error that it flourishes because it spreads from nangling to southern part. Not the intended meaning.
B. One fifth of the world’s reserves of over 6M hectares of emerald bamboo groves flourishes in China and spreads from the saturated delta of the Yangtze to the borders of the Nanling mountains in the southern part of - One fifth of world's reserves of?? changes the meaning. over 6M hectares is one fifth of world reserves. Not 1/5th of worlds 6 Mn reserves.
C. One fifth of the world’s reserves of emerald bamboo groves that are over 6M hectares and that spread from the saturated delta of the Yangtze to the borders of the Nanling mountains in the southern part of China flourish in - bamboo groves that are over 6Mn hectares?? and the second that also acts restrictive to the only those bamboo that spread... which again isn't intended.
D. Over 6M hectares of emerald bamboo groves, one fifth of the world’s reserves, spreading from the saturated delta of the Yangtze to the borders of the Nanling mountains in the southern part of China, flourish in - There are no issues here. One fifth currently means hectares spreading from nangling. Here the flourish also is also correctly modifying the 6Mn hectares of Bamboo groves. So far better of the others.
E. One fifth of the world’s reserves of over 6M hectares of emerald bamboo groves flourish in China and spread from the saturated delta of the Yangtze to the borders of the Nanling mountains in the southern part -Again one fifth of of over 6Mn changes meaning. Also this options means the groves do 2 things flourish and the spread. Which is not correct.