Encompassing nearly a third of the planet's land surface, forests perform a vital function in sustaining the conditions for human life in cities, maintaining air quality by ridding the atmosphere of excess carbon, and ensuring consistent global temperatures through the storage of greenhouse gases.
A)Encompassing nearly a third of the planet's land surface, forests perform a vital function in sustaining the conditions for human life in cities, maintaining
Starting -ing modifier correctly modifies the immediately following noun "forests". Present continuous forms of "maintain" and "sustain" are parallel with that of "ensure". No other obvious errors observed. Correct answer.B)Encompassing nearly a third of the planet's land surface and performing a vital function in sustaining the condition for human life in cities, forests maintain
There is a meaning shift in this option. The "vital function" performed by the forests is only "in sustaining the condition for human life in cities", whereas as per the question forests perform a vital function in three activities - "sustaining...", "maintaining..." and "ensuring....".C)Forests encompass nearly a third of the planet's land surface and perform a vital function in sustaining conditions for human life in cities, and by maintaining
The problem is with the usage of the word "by". In this context, "by" indicates something being used as an instrument / agency for doing another thing, which is not the meaning intended in the question. Additionally, the usage of "by" results in a sentence fragment.D)Forests encompass nearly a third of the planet's land surface, perform a vital function in sustaining the conditions for human life in cities, and maintain
There is a meaning shift in this answer option. As per the question forests perform a vital function in three activities whereas this option states the vital function to be only in "sustaining...". Also, this implies that forests "maintain..." by doing both of the following actions:
i) "ridding..." and
ii) "ensuring..."
while the question implies that forests "maintain..." by "ridding..." and "ensuring..." is a separate function. E)Forests encompass nearly a third of the planet's land surface, performing a vital function in sustaining the conditions for human life in cities, and they maintain
Same as (D)Hope this helps.