Answer
a. R2.
b. R1.
Work Step by Step
a. The current is the same when the lightbulbs are in series. The power equation, $P=I^{2}R$, says that the higher resistance bulb consumes more power, and burns brighter.
b. The voltage is the same when the lightbulbs are in parallel. Using the power equation $P=\frac{V^{2}}{R}$, we see that the lower-resistance bulb consumes more power and burns brighter.