They hardly believe that the apartment which costs them $ 4,

首先排除掉b和c,因为被形容的对象是apartment,一般形容房间的空间大小用big,或small,而不用little.
再看small是个形容词,形容房间大小的,要修饰这个形容词就要一个副词来搭配,so,very这些都可做副词,而such一般只做形容词.
要用such的话要写成such a small apartment.