What OAuth flow is best suited for this scenario?

Universal Containers (UC) uses a home-grown Employee portal for their employees to collaborate. UCdecides to use Salesforce Ideas to allow employees to post Ideas from the Employee portal. When users click on some of the links in the Employee portal, the users should be redirected to Salesforce, authenticated, and presented with the relevant pages .

What OAuth flow is best suited for this scenario?
A . Web Application flow
B. SAML Bearer Assertion flow
C. User-Agent flow
D. Web Server flow

Answer: D