The question is asking for the reason of obtaining an acidic solution. The only reason can be when ethanol follows a substitution reaction approach to substitute the bromine atom and form HBr as a sidr product. This will indeed form an acidic solution.
However, if we had gone for Elimination pathway, we can see that it would require
Lesser role of steric hindrance by adjacent methyl groups.
Resonance stabilised carbocation (if we choose E1 mechanism).
The aquous solvent will too stabilise the carbocation (although both 2,3 points also valid in SN1 reaction).
- The product form will be in conjugation with the benzene ring.
Aren't these enough for ethanol to act a bit more acidic and lead to elimination than substitution?