There are multiple splits here.
1. "as old as" vs "older than"
There is a change in meaning here. The viking colony can't be 600 years as old as the first NA colony. Its illogical. So ideally we should use "older than", which conveys the correct idea that VC is much older than first NA colony. (eliminate A, C and E)
2. supposed vs supposedly
We are clearly modifying the adjective "first" here and hence need an adverb. "supposed" is an adjective vis-a-vis "supposedly", which is an adverb. Eliminate C and D
3. Pronoun errors
"their", "its" are both wrong. We don't need plural or singular possessive pronoun here. The NA colony is distinct and not a subset of VC. Eliminate D and E