Although ice particles in the upper atmosphere benefit Earth in that

(A) acting as global thermostat and thus keeping Earth from either burning up or freezing over, they also accelerate

CORRECT: This answer choice conveys the intended meaning in a clear and idiomatically correct fashion;
Adverbial modifier -- acting as a global thermostat -- correctly modifies the clause about the ice particles' reflection and absorption of solar radiation, describing the results of those phenomena
The main clause ‘they also accelerate’ starts in the correct place, and, as expected from subordinating conjunction ‘although’, stands in conceptual opposition to the subordinate clause that begins the sentence; The subordinating conjunction ‘although’ is used to introduce a contrast BETWEEN the clause introduced by ‘although’ (subordinate clause) AND the clause following the comma (main clause) - ‘Although [subordinate clause], [main clause]’;



(B) acting as a global thermostat and thus keeping Earth either from burning up or freezing over, while also accelerating

Meaning: the placement of the preposition ‘from’ in the construction ‘keeping Earth EITHER from burning up OR freezing over’ doesn’t make sense in the given context, as it implies that preposition ‘from’ applies only to the first part of the construction; In other words, ‘Ice particles act as global thermostat and thus EITHER keep Earth from burning up OR keep Earth freezing over’; obviously, the preposition ‘from’ has to be placed before the ‘EITHER..OR’ construction, because ‘keeping Earth freezing over’ doesn’t logically qualify as a ‘benefit’: ‘… thus keeping Earth from EITHER burning up OR freezing over…’
Construction: subordinating conjunction ‘Although’ is used to introduce a contrast BETWEEN the clause introduced by ‘although’ (subordinate clause) AND the clause following the comma (main clause) - ‘Although [subordinate clause], [main clause]’. This answer choice is a sentence fragment, as it lacks the main clause and consists of one subordinate clause followed by modifiers - ‘Although [subordinate clause], [modifier], [modifier], [main clause –missing]’; furthermore, since the main clause is missing, the implication is that the modifier ‘while also accelerating the destruction of the ozone layer by reacting with chlorofluorocarbons’ is further describing the benefits of ice particles, whereas, clearly, there is nothing beneficial in the destruction of ozone layer;


(C) act as a global thermostat and thus keep Earth from either burning up or freezing over, while also accelerating

Meaning/Construction: This answer choice makes no grammatical sense; it seems to present a list of benefits, but with complete disregard to parallelism and punctuation: ‘reflect and absorb..., act as X, and thus..., while also accelerating ....’; furthermore, this answer choice is a sentence fragment, as it lacks the main clause and consists of one subordinate clause followed by a long modifier - ‘Although [subordinate clause], [modifier], [main clause –missing]’; furthermore, since the main clause is missing, the implication is that the modifier ‘while also accelerating the destruction of the ozone layer by reacting with chlorofluorocarbons’ is further describing the benefits of ice particles, whereas, clearly, there is nothing beneficial in the destruction of ozone layer;


(D) they act as a global thermostat that thus keeps the Earth either from burning up or freezing over even though it also accelerates

Meaning: the subordinating conjunction ‘Although’ is used to introduce a contrast BETWEEN the clause introduced by ‘although’ (subordinate clause) AND the clause following the comma (main clause) - ‘Although [subordinate clause], [main clause]’; Clearly, there is no apparent contrast between the subordinate clause (Although ice particles benefit Earth in that they reflect and absorb solar radiation) and the main clause (they act as a global thermostat)’; on the contrary, the ‘global thermostat’ idea flows smoothly from the ice particles' reflection and absorption of solar radiation.
Unidiomatic: construction ‘thermostat that THUS keeps’ is unidiomatic, the ‘thus’ is completely out of place and unnecessary;
Meaning: the placement of the preposition ‘from’ in the construction ‘keeps the Earth EITHER from burning up OR freezing over’ doesn’t make sense in the given context, as it implies that preposition ‘from’ applies only to the first part of the construction; In other words, ‘Ice particles act as global thermostat and thus EITHER keep Earth from burning up OR keep Earth freezing over’; obviously, the preposition ‘from’ has to be placed before the ‘EITHER..OR’ construction, because ‘keeping Earth freezing over’ doesn’t logically qualify as a ‘benefit’;
Awkward: aside from using a contrasting conjunction ‘although’ without apparent contrast, this answer choice adds another contrasting conjunction ‘even though’, thereby creating an even more awkward sentence;


(E) they act as a global thermostat to thus keep Earth from either burning up or freezing over, but they also accelerate

Meaning: the subordinating conjunction ‘Although’ is used to introduce a contrast BETWEEN the clause introduced by ‘although’ (subordinate clause) AND the clause following the comma (main clause) - ‘Although [subordinate clause], [main clause]’; Clearly, there is no apparent contrast between the subordinate clause (Although ice particles benefit Earth in that they reflect and absorb solar radiation) and the main clause (they act as a global thermostat)’; on the contrary, the ‘global thermostat’ idea flows smoothly from the ice particles' reflection and absorption of solar radiation.
Meaning: construction ‘they (ice particles) act as thermostat TO THUS keep Earth…’ nonsensically implies that ‘ice particles intentionally act as thermostat to protect the earth from extreme temperatures’, obviously ‘ice particles are inanimate objects that don’t have free will, not to mention intentions’
Awkward: aside from using a contrasting conjunction ‘although’ without apparent contrast, this answer choice adds another contrasting conjunction ‘but’, thereby creating an even more awkward sentence;