Updating Dataverse records by alternate key with UpdateRequest and UpsertRequest

Alternate keys let you perform updates (and upserts) without the primary key. UpdateRequest updates the matching row; UpsertRequest updates it if it exists (or inserts if not) — both resolve the target via the alternate key, which is what the scenario requires.

When a table has an alternate key, you can address an existing row by that key instead of its primary key. Both UpdateRequest and UpsertRequest accept the alternate-key values to locate the row and apply the update, so either request satisfies the requirement.

Choosing CreateRequest (C) — it inserts a new row rather than updating an existing one, so it does not meet an 'update existing records' requirement. RetrieveRequest (D) only reads, it does not update.