The inability to remember whose face appears on a five-dollar bill is mostly likely

encoding

Explanation:

According to my research on studies conducted by various neurologists, I can say that based on the information provided within the question this is most likely due to a failure in encoding. Encoding is known as the first step when creating a new memory, it changes the item that you are observing into data that is stored in the brain to later be remembered from short-term or long-term memory. Such an example is the one in the question, where you are looking at a five-dollar bill and converting the data into memory, which is what most likely failed in this situation.

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