The validation belongs on the Opportunity Product because the discount being restricted is a value associated with each individual product line. A validation rule evaluates the record being saved and prevents the transaction when its formula evaluates to TRUE. Therefore, the rule can evaluate the Opportunity Product discount and generate an error whenever it exceeds 15%. Putting the validation on Opportunity would target the parent record rather than the specific line item where the invalid value occurs. An approval process is unnecessary because the requirement is to prevent the save, not route an exception for managerial approval. A Big Deal Alert applies to large Opportunities and is unrelated to product-level discount enforcement. Salesforce recommends validation rules for enforcing record-level business data requirements before save.

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