Which of the following must occur for an unfiled federal tax lien to be valid against a purchase?

For an unfiled federal tax lien to be valid against a purchase, the requirement of filing notice thereof is essential. A federal tax lien attaches automatically to a taxpayer's property when the IRS assesses a tax and the taxpayer neglects or refuses to pay the assessed amount. However, for the lien to be enforceable with respect to third parties, such as buyers of real estate, it must be officially filed in accordance with statutory requirements, typically in the public records.

Filing notice creates a public record that informs potential buyers and other creditors of the federal tax obligation attached to the property. This ensures that all interested parties are alerted to the existence of the lien, thereby providing legal notice which is crucial for the validity of the lien against subsequent purchasers. Without this filing, the lien may not hold up against a purchaser who buys the property without knowledge of the lien's existence.

In contrast, the other options either do not pertain to the validity of the lien against purchases or are irrelevant to the necessary legal process that protects the rights of the creditor in the event of a sale.