Answer:
Acid
Step-by-step explanation:
Acid pHs range from 1 to 6. Base pHs range from 8 to 14. 7 is neutral.
The smaller the pH, the more acidic the substance is.
Right now, the solution with pH 5 is moderately acidic. However, if its pH decreases to 3, that means the solution has become much more acidic. The only way to become more acidic is to have added an acid.
Neutralizer is wrong because it would not make the acid more acidic; it would probably neutralize it and either keeps its pH the same or raise its pH a litte.
Base is wrong because if a base was added, the pH would likely increase.
There is enough info to tell.
Hope this helps!