Explanation
You are not charged for unused network interfaces. Therefore, deleting unused network interfaces will not reduce the Azure costs for the company.
References:
https://docs.microsoft.com/en-us/azure/advisor/advisor-cost-recommendations#reduce-costs-by-deleting-or-recon

Section: Describe Core Azure Services
Explanation:
Box 1: No
You can assign service administrators and co-administrators in the Azure Portal but there can only be one account administrator.
Box 2: No
You need an Azure Active Directory account to manage a subscription, not a Microsoft account.
An account is created in the Azure Active Directory when you create the subscription. Further accounts can be created in the Azure Active Directory to manage the subscription.
Box 3: No
Resource groups are logical containers for Azure resources. However, resource groups do not contain subscriptions. Subscriptions contain resource groups.
Reference:
https://docs.microsoft.com/en-us/office365/enterprise/subscriptions-licenses-accounts-and-tenants-for- microsoft-cloud-offerings

Section: Understand Cloud Concepts
Explanation:
One of the major changes that you will face when you move from on-premises cloud to the public cloud is the switch from capital expenditure (buying hardware) to operating expenditure (paying for service as you use it).
This switch also requires more careful management of your costs. The benefit of the cloud is that you can fundamentally and positively affect the cost of a service you use by merely shutting down or resizing it when it's not needed.
References:
https://docs.microsoft.com/en-us/azure/architecture/cloud-adoption/appendix/azure-scaffold

Section: Understand Core Azure Services