Introduction:
The barter system refers to the exchange of goods and services without the use of a medium of exchange, such as money. In this system, goods and services are directly traded for other goods and services. However, over time, the barter system has been replaced by the monetary system, which uses money as a medium of exchange.

Explanation:
The correct answer to the question is option 'b' - the monetary system. The monetary system refers to the use of money as a medium of exchange for goods and services. Money serves as a common measure of value, making transactions easier and more efficient. There are several reasons why the barter system has been replaced by the monetary system:

1. Difficulties in Exchange:
In a barter system, individuals would have to find someone who has the desired goods or services they need and is willing to trade for what they have. This process can be time-consuming and inefficient, as it requires a double coincidence of wants. The use of money eliminates these difficulties, as it can be universally accepted as a medium of exchange.

2. Lack of Standardization:
In a barter system, the value of goods and services is subjective and can vary from person to person. There is no standard measure of value, which makes it challenging to determine fair exchanges. Money, on the other hand, provides a standardized measure of value, allowing for more accurate and efficient transactions.

3. Storage and Portability:
With the barter system, individuals need to physically possess the goods or services they wish to trade. This can be challenging, especially when dealing with perishable goods or large items. Money, being a portable and divisible asset, can be easily stored and transported, making transactions more convenient.

4. Economic Growth:
The monetary system promotes economic growth by facilitating specialization and division of labor. In a barter system, individuals are limited to what they can produce or provide directly. With money, individuals can specialize in a particular field and exchange their goods or services for money, which can then be used to acquire other goods and services. This promotes efficiency and productivity, leading to economic growth.

Conclusion:
In conclusion, the barter system has been replaced by the monetary system due to the difficulties in exchange, lack of standardization, storage and portability issues, and the promotion of economic growth. The monetary system, using money as a medium of exchange, has provided a more efficient and convenient way to facilitate transactions and foster economic development.