Answer: Real estate
Explanation: Liquidity refers to the ability of a security to be converted into cash without having a major change in its price. In other words, liquidity is the relationship between the speed of sale of a security and its change in price.
Real estate refers to the land or other housing facilities etc. These assets require huge amount for purchase. It takes to find a buyer willing to make such a big investment, thus, they are not liquid.
Hence from the above we can conclude that the right option is C.