Answer
a. parallel
b. (I)
Work Step by Step
(a) We know that the power dissipated by the each bulb is given as $ P=\frac{V^2}{R}$. In a parallel connection, the potential difference is the same, and there is a smaller equivalent resistance than when in series; hence, parallel lights have a greater output and the lights are brighter when connected in parallel.
(b) We know that the best explanation is option (I) -- that is, when connected in parallel, each bulb experiences the maximum emf and dissipates the maximum power.
