正确答案为：How long has the underground in Nanjing been in service?
或：Since when has the underground in Nanjing been in service?
或：Since what time has the underground in Nanjing been in service?
1.疑问句中,助动词has应该提前；
2.对since...提问,可以像对for...提问一样,都用How long作为疑问词,当然也可以用since when或since what time.