▲最佳答案应是 when，因为这里是针对时间提问。类似的例句如：

When was the last time you made a cake?  你上次做蛋糕是什么时候？

When was the last time you had a cigarette? 你上一次吸烟是在什么时候？

When is the last time you had a tetanus shot? 你上次打破伤风针是什么时候？

When was the last time you listened to an album all the way through? 你上一次完整地听完一张专辑是什么时候？

When was the last time you cleaned the dog kennel? It stinks to high heaven.你都多久没打扫狗窝了？已经臭气燻天了。

▲比较陈述句的用法：

Christmas was the last time we had a party. 我们上一次搞聚会是圣诞节的时候。

I haven’t used my camera recently. June was the last time I took a photo. 我最近一直没有用相机。我上次照相是在六月份的时候。

比较：

When was the last time you saw him? 你上次见她是什么时候？

Monday was the last time I saw him. 我上次见到他是星期一。