During which period caste system was really a Varna system and consisted of only four major divisions?

Late Vedic Period

The correct answer is Option 4: Late Vedic Period

NCERT: The varna classification is roughly three thousand years old. However, the ‘caste system’ stood for different things in different time periods, so that it is misleading to think of the same system continuing for three thousand years. In its earliest phase, in the late Vedic period roughly between 900 — 500 BC, the caste system was really a varna system and consisted of only four major divisions. These divisions were not very elaborate or very rigid, and they were not determined by birth. Movement across the categories seems to have been not only possible but quite common.