That is photo when he was -----he was in the army

应选B。
That is photo when he was in the 20s when he was in the army。
第一个“when”引导的是状语从句；
20s表示1世纪20年代,前面不能出现形容词形物主代词；
第二个“when”引导的是定语从句,它修饰“20s”。修饰一个时间的定语从句可以用关系副词“when”引导,也可以用“介词+which”引导,不能用“that”。
全句翻译：这是他一世纪二十年代参军时的照片。