Which investment has the least liquidity?

The investment with the least liquidity is B. House.

Liquidity refers to how easily an asset can be converted into cash without significantly affecting its price. A house takes time to sell and usually involves various costs, making it less liquid compared to a checking account (which can be accessed immediately), a mutual fund (which can often be sold relatively quickly), and a small business (though it can vary, selling a business typically takes more time as well).