Naomi hides the original data in a Social Security number field to make sure that it is not exposed to users of her database. What data security technique does this describe?

Masking is a technique that alters content to conceal personally identifiable information (PII) or other sensitive data.
On the other hand, tokenization replaces sensitive information with a non-sensitive alternative that enables the data to be processed in a useful manner.
Encryption, which would require the data to be decrypted to be used, is not mentioned in this context.
Hashing is another approach that can be used to conceal values, but it does not preserve the ability to work with the data.