A bakery only accepts R100 notes for purchases, even when customers buy a loaf of bread worth R20. Which key characteristic of money is being violated in practice?

• Durability: Money must be able to withstand wear and tear.
• Divisibility: Money must be easily divisible into smaller units to facilitate transactions of varying values.
• Acceptability: Money must be generally accepted as a medium of exchange by everyone in the economy.
• Portability: Money must be easy to carry around.

The bakery's policy means that a customer cannot use a R20 note for a R20 loaf of bread, nor can they easily make an exact R20 payment with a R100 note if the bakery doesn't provide change or accept smaller denominations. This makes it difficult to conduct transactions for small amounts.

The inability to use money in smaller denominations for smaller purchases directly violates the characteristic of divisibility. Money should be able to be broken down into smaller units to match the value of goods and services being exchanged.

The final answer is $\boxed{\text{B. Divisibility}}$.

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