a. D; b. B; c. At A.
a. Equivalence point
The equivalence point is the point at which the pH changes most rapidly with the addition of a small amount of base.
It is also the inflection point on the graph: the point at which the curvature of the graph changes from concave up to concave down.
Point D is the equivalence point.
b. Buffering point
The titration curve shown has two parts: ABCD, in which the graph curves up, and DE, in which the graph curves down.
The titration curve for a weak acid has three parts: AB (curving down), BCD (curvature up), and DE (curvature down).
(Adapted from ThoughtCo)
Point B, where the curvature changes from down to up, is the flattest part of the curve.
It appears when the acid is half-neutralized (the half-equivalence point, 12.5 mL).
This is the point at which the pH changes most slowly when more base is added, that is, the point at which the solution is best buffered against changes in pH.
c. pH curve for stronger acid
Say you had 1 mol/L solutions of two strong acids, HX and HY.
Each acid would be completely dissociated into hydronium ions, so each solution would have the same pH.
The titration curves for each acid would be identical.
They would each intersect the y-axis at Point A.