My sister Gina bought a 1957 Chevy at the car auction. It was old and rusty.
or
My sister Gina bought a 1957 Chevy at the car auction; it was old and rusty.
or
My sister Gina bought a 1957 Chevy at the car auction, and it was old and rusty.
Good luck!
I would say the answer is C.
