きのうの パーティー＿＿＿何を しましたか。

[Keito4]

I thought the right answer would be で, but it appeared to be を... Why is that so? 教えてください。

[furrykef]

I'm pretty darn sure it's supposed to be で. Where did this sentence come from? If it's a quiz, maybe they just made a simple mistake?

[Yudan Taiteki]

There's no way it can be を; you are not allowed to have two を particles in one phrase.

昨日のパーティーを何にしましたか is fine, and so is 昨日のパーティーで何をしましたか (meaning different things), but you can't have both particles be を.