あした__________べんきょうします。
• X
• を
• に

The hint to this question is あした "tomorrow."

を - indicates an action acting upon an object

e.g りんごをたべます - eat an apple.

But あした is not an object, it's a concept of time.

に - indicates position, direction, as well as a specific time

e.g ぎんこうにいきます - go to the bank
e.g ２じにべんきょうします - study at 2:00

But the trick is あした is not a specific time.

You do not use a particle with non-specific times such as:
ごぜん - morning
ごご - afternoon
きょう - today
あした - tomorrow
きのう - yesterday
こんしゅう - this week
らいしゅう - next week
せんしゅう - last week
こんげつ - this month
らいげつ - next month
ことし - this year
らいねん - next year

A specific time would specify an exact time or date. In those situations you use に.