Which administrative release pertains to proposed transactions rather than completed transactions?

The correct choice is a Letter Ruling, which provides guidance on proposed transactions rather than transactions that have already been completed. A Letter Ruling is an official interpretation issued by the IRS in response to a taxpayer's request regarding the tax implications of a specific situation or transaction before it occurs. This is particularly valuable for businesses and individuals seeking certainty about how the tax laws will apply to their intended actions.

The main purpose of a Letter Ruling is to address the expected tax treatment of a transaction, allowing taxpayers to plan accordingly and comply with the law. This preemptive guidance helps to minimize risks associated with unforeseen tax liabilities from now-completed actions.

In contrast, the other administrative releases mentioned—Technical Advice Memorandum, Determination Letter, and Field Service Advice—typically relate to completed transactions or situations that have already occurred, where an official ruling or interpretation from the IRS is requested or necessary. Thus, they do not serve the same function as a Letter Ruling in terms of addressing proposed transactions.