1. some time意思是“一段/些时间”(表时间段),而非sometime所表达的“(过去或将来的)某个时刻”.
如:I'll call you sometime next week.
I saw your sister in Shanghai sometime last week.
I saw your sister in Shanghai some time ago.
2. 确如楼主所言,如改成How about sometime next Monday in my office?就对了.
3. 而选项B中的some other time是个固定短语,意思是“改天吧”. http://www.***.com/some%20other%20time/