4. at what point was the solution in beaker ""b"" neutralized?

Explanation:

This is called the equivalence point. That is the exact point during titration where the relative amounts of each substance in the chemical reaction of titration become well defined by the coefficients in the balanced chemical equation used for expressing the kinetics of reaction.  

A titration is an experimental set up   with a controlled acid-base neutralization reaction.it is used to determine the unknown concentration of an acid or a base.  

Acids and bases neutralize each other, forming a salt and water. Basically, a strong acid-strong base neutralization gives  a neutral solution with a pH of 7.

b is the correct answer