a) A is sending packet to B:
Here Switch is used to connect them.
Switch doesn't changes the MAC address in the network.
So When the packet is received , it have $\color{purple} {Source \: as \: MAC \:of \:A}$
b)A is sending packet to C:
Here Router is involved to connect A and C.
At A:
, We have Source MAC as A's MAC and Destination Mac as R1's MAC
When it reaches R1, Router change MAC addresses like
When this is received at C ,we will have $\color{blue} {\:Source\: as\: MAC\:of\: Router}$