答案是if虚拟条件句的省略,相当于If they were to be painted, the wall would be completely dry by tomorrow evening. 表示跟将来事实相反.if虚拟条件从句中如出现were,had,或者should,可提前至主语之前,省略if.
你选的after painting them,是省略了主语和be动词,这种省略的前提是从句和主句的主语是一致的,所以其完整形式应为：After the wall was painting them,这样可以看出句意不顺.