What Oauth flows should be considered to support this requirement?

Universal Containers (UC) wants to build a custom mobile app for their field reps to create orders in salesforce. After the first time the users log in, they must be able to access salesforce upon opening the mobile app without being prompted to log in again.

What Oauth flows should be considered to support this requirement?
A . Web Server flow with a Refresh Token.
B. Mobile Agent flow with a Bearer Token.
C. User Agent flow with a Refresh Token.
D. SAML Assertion flow with a Bearer Token.

Answer: C