It may not be correct to look at this issue in active or passive voice context. After all passive voice is not taboo in GMAT
Besides, there is virtually no choice without the use of passive voice.
‘Are known’ in A, ‘are known’ and ‘has been observed’ in B, and C, ‘is observed’ in D and E, are all in passive voice.
Another point is that the use of once in choices B and C does not gel well within the meaning. Once may be used to mean as soon as or at that very moment. In a remote sense, it may also mean that some people require seeing such cosmic phenomena just one time and then they are inclined to stoke their dormant faiths.
All these are general observations and generalizations. The use of present tense is acceptable in such instances, IMO.
Ignoring the passive voice insinuation, I would rather prefer A
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