a
In the titration of weak acid \((CH_3COOH)\) with a strong base \((NaOH)\), there is a change in the \(pH\) value at the end point. But it is not sharp due to weak ionisation of \(CH_3COOH\). Due to the excess of free base beyond the equivalence point, there is steep rise in \(pH\) which is indicated by vertical line. Hence, the vertical line in the graph indicates alkaline nature of equivalence.