Which blade should you instruct the finance department auditors to use?

Cost analysis: Correct Option

In cost analysis blade of Azure, you can see all the detail for custom time span. You can use this to determine expenditure of last few day, weeks, and month. Below options are available in Cost analysis blade for filtering information by time span:  last 7 days, last 30 days, and custom date range. Choosing the first option (last 7 days) auditors can view the costs by time span.

Cost analysis shows data for the current month by default. Use the date selector to switch to common date ranges quickly. Examples include the last seven days, the last month, the current year, or a custom date range. Pay-as-you-go subscriptions also include date ranges based on your billing period, which isn't bound to the calendar month, like the current billing period or last invoice. Use the  links at the top of the menu to jump to the previous or next period, respectively. For example, 

Invoice: Incorrect Option

Invoices can only be used for past billing periods not for current billing period, i.e. if your requirement is to know the last week's cost then that also not filled by invoices because Azure  generates invoice at the end of the month. Even though Invoices have custom timespan, but when you put in dates for a week, the pane would be empty. Below is from Microsoft document:

Resource Provider: Incorrect Option

When deploying resources, you frequently need to retrieve information about the resource providers and types. For example, if you want to store keys and secrets, you work with the Microsoft.KeyVault resource provider. This resource provider offers a resource type called vaults for creating the key vault. This is not useful for reviewing all Azure costs from the past week which is required for audit.

Payment method: Incorrect Option

Payment methods is not useful for reviewing all Azure costs from the past week which is required for audit.