Despite the fact that the impact of Daylight Saving Time, at the onset of which clocks are set forward by an hour, have been shown to be neutral, at best—research indicates that it may actually increase the number of car accidents occurring on the Monday after the clocks spring forward—some countries do not observe it.
A - Despite the fact that the impact of Daylight Saving Time, at the onset of which clocks are set forward by an hour, have been shown to be neutral, at best—research indicates that it may actually increase
The subject of the first clause (subordinating clause) is 'impact', which is a singular noun. so we need a singular verb but this option has a plural verb 'have'. Moreover usage of 'despite' is wrong as the sentence doesn't show a contrast.
B - Even though the impact of Daylight Saving Time, at the onset of which clocks are set forward by an hour, has been shown to be neutral, at best—research indicates that it actually increases
same issue that we saw in the first option. SVA problem
C - Since the impact of Daylight Saving Time, at the onset of which clocks are set forward by an hour, has been shown to be neutral, at best—research indicates that it actually increases
no SVA problem. pronoun 'it' refers to the subject in the first clause 'impact'. so no grammatical error. since is used to show a reason.so no meaning issues in the sentence.
D - Because the impact of Daylight Saving Time, at the onset of which clocks are set forward by an hour, has been shown to be neutral—at best, research indicates that it may actually result in an increase in
This sentence doesn't convey the meaning effectively. it should be 'research indicates that it actually increases' not research indicates that it may actually result in an increase in' GMAT prefers a verb over noun(VAN principle,verb,adjective,noun)
E - Because Daylight Saving Time, at the onset of which clocks are set forward by an hour, has been shown to have a neutral impact, at best—research indicates that it may actually result in a possible increase in
'may' and 'possible' conveys same idea, so redundant.