she couldn't even rent a cheapest car,----- this one

A
let alone更不用说.后面接的词或短语应当与前面相应的词或短语形式一样.本题前面的动词是rent,是接在could后面的动词原形,所以后面也填一个原形的形式.
let alone=to say nothing of=not to speak of=not to mention