Actually, I'm not sure why (A) is not correct.
A has at least a couple of flaws in this sentence:
1. it...its... is redundant
2. the spread (positioning) between unlikely and significant subtlely alters the author's intended meaning. (E) stylistically and syntactically places ...not likely to be significant... in the clearest tone.
3. At face value, IMHO, based on empirical observations of 1000s of SCs
, the GMAT tends to
prefer not + adj over un+adj constructions.