Extra exam question E7D07

WhyIn the circuit shown in Figure E7-2, C2 bypasses rectifier output ripple around D1. In Figure E7-2, C2 is connected across D1, the Zener reference diode, and bypasses rectifier output ripple around D1, keeping ripple on the unregulated input from disturbing the clean, stable reference voltage that sets the regulator's output, answer A. It isn't a brute-force output filter (that role, if present, would sit at the actual output), isn't there to prevent self-oscillation, and doesn't provide fixed DC bias for Q1 by itself, that's the combined job of the R1/D1 reference network.