In order to keep the economy stable, Han emperors

Introduced the first Chinese coins.
Preventing merchants from selling goods outside of China.
Bought and stored good to avoid shortages.
Banned monopolies in major industries.

All of these options contributed to keeping the economy stable during the reign of the Han emperors in ancient China. Let's discuss each option in more detail:

1. Introduced the first Chinese coins: The introduction of standardized coinage provided a medium of exchange that facilitated trade and commerce within the empire. It allowed for easier transactions, eliminated the need for bartering, and boosted economic activities.

2. Preventing merchants from selling goods outside of China: While it is true that the Han government implemented restrictions on foreign trade, this was mainly aimed at controlling and regulating the flow of goods entering and leaving the empire. By doing so, they aimed to protect domestic industries and prevent excessive outflow of wealth. However, this measure had minimal impact on the overall stability of the economy.

3. Bought and stored goods to avoid shortages: The Han emperors recognized the importance of maintaining stable supplies of essential goods. They implemented policies to purchase and store surplus grains and other vital commodities during times of abundant production. This strategic stockpiling helped the government to alleviate shortages during periods of crop failures, natural disasters, or other emergencies, thereby maintaining stability in the economy.

4. Banned monopolies in major industries: The Han dynasty recognized the negative impact of monopolies on the economy. To prevent the concentration of power and ensure fair competition, they implemented regulations that prohibited monopolistic practices in major industries. This allowed for a more level playing field, fostering innovation, and preventing the exploitation of consumers, ultimately contributing to economic stability.

In summary, while preventing merchants from selling goods outside of China might not have directly contributed to economic stability, the introduction of Chinese coins, strategic stockpiling of goods, and banning monopolies were important measures implemented by the Han emperors to ensure stability in the economy.