Outdated question, user-agent flow is not supported anymore.

The correct answer is D.
User-agent oAuth flow is recommend for the security purpose.
https://help.salesforce.com/s/articleView?id=sf.remoteaccess_oauth_user_agent_flow.htm&type=5

Username-Password flow is not recommended because of the security risks. Mobile app can save the user credential.
https://help.salesforce.com/s/articleView?id=sf.remoteaccess_oauth_username_password_flow.htm&type=5

Answer is B.
The oAuth Username-Password flow is an appropriate solution for mobile applications that need to authenticate with Salesforce. This flow enables a user to provide their username and password, which the mobile application can use to obtain an oAuth token. The oAuth token can then be used for subsequent API calls to access and update data in Salesforce.


Option A is not recommended because it requires the mobile application to handle the user's username and password, which is a security risk. Option C is also not recommended as it requires the storage of sensitive information within the application code. Option D is not appropriate for this use case because it requires the user to interact with a separate browser to authenticate, which is not ideal for a mobile experience.