Step 1: Understand the money market.
The money market is a segment of the financial market that deals with short-term borrowing and lending. One of its primary challenges is the availability of capital, which affects the liquidity and efficiency of the market.
Step 2: Evaluate the options.
- Lack of capital is a key issue that restricts the operations and growth of the money market, especially during periods of economic downturn.
- Hundi is a traditional method of informal credit, which is not a core problem of the money market itself.
- Lack of market typically refers to less demand or liquidity, but capital availability is a more pressing issue.
Step 3: Conclude.
The main problem of the money market is the lack of capital.
Final Answer:
\[
\boxed{\text{Lack of capital}}
\]