The correct answer is D, as Cloud Billing supports exporting billing data in flat files, specifically CSV and JSON formats. Given the billing department's preference for Excel, CSV is the ideal choice since Excel can directly open CSV files without the need for conversion. Options A, B, and C are incorrect because they do not directly support Excel and would require additional steps for conversion. For more details, refer to the GCP documentation on Cloud Billing Export Billing Data, which explains how to export daily usage and cost estimates to a CSV or JSON file stored in a specified Google Cloud Storage bucket. This data can then be accessed via the Cloud Storage API, CLI tool, or Google Cloud Platform Console, with usage data labeled by project number and resource type, and access controlled via ACLs on your Cloud Storage bucket.