Comprehensive and Detailed Explanation:

Completeness in invoicing means that all services delivered (electricity consumed) are properly billed. Therefore, the most effective approach is directly comparing consumption data with billed amounts (D) to identify unbilled usage. Options A and C provide insight into payment behavior or consumption trends but do not test billing completeness. Option B (ratio analysis) could provide high-level patterns but would not pinpoint specific missing invoices. The most reliable procedure is a one-to-one reconciliation between metered consumption and invoicing data, as it ensures every unit of electricity delivered is accounted for in billing. This method aligns with IIA Standard 2310 – Identifying Information, which requires sufficient, relevant, and reliable evidence to support audit conclusions. By directly comparing inputs (consumption) and outputs (invoices), the auditor can identify systemic gaps, errors, or fraud in the billing process.