Reference:
https://docs.microsoft.com/en-us/azure/virtual-network/virtual-network-peering-overview
https://docs.microsoft.com/en-us/azure/networking/networking-overview#internet-connectivity

Assign a role to a user
Sign in to the Azure portal with an account that's a global admin or privileged role admin for the directory.
Select Azure Active Directory, select Users, and then select a specific user from the list.
For the selected user, select Directory role, select Add role, and then pick the appropriate admin roles from the Directory roles list, such as Conditional access administrator.
Press Select to save.

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 <PREVIOUS and NEXT> links at the top of the menu to jump to the previous or next period, respectively. For example, <PREVIOUS will switch from the Last 7 days to 8-14 days ago or 15-21 days ago.

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.
Reference:
https://docs.microsoft.com/en-us/azure/cost-management-billing/costs/quick-acm-cost-analysis
https://docs.microsoft.com/en-us/azure/cost-management-billing/manage/download-azure-invoice-daily-usage-date

URL Path Based Routing allows you to route traffic to back-end server pools based on URL Paths of the request.
In the question there are two different path from where the traffic is getting generated as below
https://adatum.com/internal
https://adatum.com/external
So in this case we can use URL path-based routing feature of Application Gateway.

Reference:
https://docs.microsoft.com/en-us/azure/application-gateway/url-route-overview

Reference:
https://docs.microsoft.com/en-us/azure/active-directory/authentication/quickstart-sspr
https://docs.microsoft.com/en-us/azure/active-directory/authentication/active-directory-passwords-faq