Explanation/Reference:
Explanation:
First we should check Branch3 (and R1) with the "show ip interface brief" command to find any Layer1/ Layer2 issue.

We see interfaces connecting between them are in "up/down" states which indicates a Layer 2 issue so we should check the configuration of these interfaces carefully witch the "show running-config" command and pay attention to these interfaces.

and on Branch3:

We learn from above config is R1 is using CHAP to authenticate Branch3 router (via the "ppp authentication chap" command on R1). Branch3 router is sending CHAP hostname "Branch_3" and CHAP password "Branch3_Secret!" to R1 to be authenticated. Therefore, we should check if R1 has already been configured with such username and password or not with the "show running-config" command on R1:

On R1 we see the configured username is "Branch3", not "Branch_3" so the usernames here are mismatched and this is the problem -> Answer A is correct.