You are the global administrator of an Azure tenant on the free tier. You have been tasked with securing the tenant with the following requirements: Requiring all users to register for Azure AD Multi-Factor Authentication.
Requiring administrators to perform multi-factor authentication.
Blocking legacy authentication protocols.
Requiring users to perform multi-factor authentication when necessary.
Protecting privileged activities like access to the Azure portal.
The solution must be as cost effective as possible.

Answer options:

A.Upgrade the tenant to Premium 1 and create conditional access policies to address the requirements.
B.Create identity protection policies to address the requirements.
C.Enable Security Defaults.
D.Upgrade the tenant to Premium 2, create conditional access and identity protection policies to address the requirements.

Correct Answer: C 
Security Defaults is a set of security mechanisms that help protect your organization from common identity-related attacks like password spray, replay, and phishing. It addresses all the requirements in this scenario. The feature is available free of charge and can therefore be activated on the free tier: 

Option A is incorrect. This would address the requirements but it will require you to purchase a premium Tier license for your tenant. Security Defaults is free of charge. Therefore this is not the most cost-effective alternative.
Option B is incorrect. Creating identity protection policies alone would not address the requirements. Azure identity protection also requires the Azure P2 tier.
Option D is incorrect. This would address all the requirements but is also the most expensive solution.
Reference: 
To know more about Security Default, please refer to the link below: 

https://docs.microsoft.com/en-us/azure/active-directory/fundamentals/concept-fundamentals-security-defaults