Answer:
The balanced single displacement reaction between aluminum and copper sulphate is as follows.

Explanation:
The given ions are as follows.
- Aluminium ion and copper sulfate ion.
Single displacement reaction occurs one element replaces another element.
Hence, the balanced chemical reaction is as follows.

<u>Oxidation half reaction:</u>

Aluminium loses three electrons.
<u>Reduction half reaction:</u>
<u>
</u>
Copper gain three electrons.
Answer is: the oxidation number of oxygen must decrease.
Oxidizing agent is element (substance) that gain electrons in chemical reaction.
For example, balanced chemical reaction: C + O₂ → CO₂.
Oxygen change oxidation number from 0 to oxidation number -2 (reduction) and carbon change from 0 to +4 (oxidation)
Answer : Bromine
Step by step: Bromine