Because his partner had to go to Sydney, Mr. Zazueta had to finish work on the adverting campaign on（ ）.
on one's own 表示独立地，主动地。由于他的伙伴去了悉尼，Zazueta 先生 不得不独立地完成广告宣传工作。固定搭配，选择 A 选项。