At present she's focused on next

At present she's focused on next month（ ）will see her set on her biggest UK tour to date.
A、what
B、it
C、when
D、that
【正确答案】：D
【题目解析】：关系分句考察。分句will see her set...缺少主语，且先行词是next month，故用关系代词that.