The Pythagoreans discovered irrationals in about the 6th century BC. However, from your options, none correspond directly to that. The closest slightly more accurate phrasing would be a broader generalization, which shows that this fall under a period of early Greek mathematics, often associated with the 5th century BC.
Is it possible there was a mistake in the options? If you meant to ask in a 6th century context, it would probably best fit a categorization close to C (7).