GMAT Sentence Correction (SC) | Re: The existence of the soul, a controversial philosophic idea, has been

GMATNinja , Can you please help here to understand how A is correct?

Choice C and E are definitely out. In choice C, its refers to all and in choice E, their modifies every.

Grammatically, All other seems OK.

Also I have a doubt in the given, non-underlined sentence -

The existence of the soul, a controversial philosophic idea, has been the subject of various arguments
This appears to be a complete independent clause, don't we need conjunction to join this with another clause?
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