During a titration of aqueous HCl with aqueous NaOH using phenolphthalein, which observation most clearly indicates that the end point has been reached?
a) A deep magenta color remains uniformly throughout the solution.
b) The pH of the solution reads exactly 7.00 on a pH meter.
c) A very pale pink color persists uniformly throughout the solution for at least 30s after swirling.
d) The solution momentarily flashes bright pink after each drop of titrant but quickly turns colorless again.