• purchases
• halo_code
• request_tag
• user_id

The correct answer is: user_id

Explanation: The user_id field contains the pseudonymous ad identifier associated with a given impression (in AMC tables such as dsp_impressions). It is frequently used to calculate unique reach and de-duplicate audience counts.

Reference content: From the official Amazon documentation (DSP impressions table):

“user_id: Pseudonymous identifier that connects user activity across different events. The field can contain ad user IDs (representing Amazon accounts), device IDs, or browser IDs. … It is often used within Common Table Expressions (CTEs) for aggregation purposes such as COUNT(DISTINCT user_id) to calculate unique reach volumes.”