The correct answer is option 4, All the above. The custom of Sacrificial-fire, also known as Yajna or Homa, was followed by not only the Vedic Aryans but also by the Romans, Greeks, and Iranians.

1. Romans: The Romans had their own version of sacrificial fire called the Vesta, where a flame was continuously kept alive by the Vestal Virgins.

2. Greeks: The Greeks practiced the custom of sacrificial fire in their religious ceremonies and rituals. They believed that offering sacrifices to the gods through fire would bring them blessings and favor.

3. Iranians: The Iranians, particularly the ancient Persians, practiced a similar form of sacrificial fire called the Yasna. Fire was considered sacred and played a central role in their religious beliefs and rituals.

Therefore, it can be concluded that the custom of sacrificial fire was followed by the Romans, Greeks, and Iranians, in addition to the Vedic Aryans.