Which of the following answers is the best answer regarding access control.

a$.$

Access control to tenant's data can only be done by the cloud.

b$.$

Access control to tenant's data cannot be done by in an untrusted cloud.

c$.$

In a trusted cloud, access control can be done using the Open Authentication $($OAuth$)$ protocol.

d$.$

Group$-$level access control in an untrusted cloud can be done using proxy re$-$encryption.