B 是正确的.
句子本意是：but he found it difficult to find【a job 】without working 但是前面已经出现过a job ,为了避免重复,用one来代替.
英语中常用one 代替上文出现的单数可数名词,用ones 代替上文出现的复数名词.
例如：The big boxes are heavier than the small ones.(ones 指boxes )