Answer
Bolt A occurred first according to the ground observer.
Work Step by Step
Refer to Figure 37.5A.
If the bolts are simultaneous to the train observer, that means that light pulses from A’ and B’ arrived at O’ simultaneously.
To the ground observer, the light from A’ traveled a longer distance to get to the train observer than the other bolt did.
Since they arrived at the same time at the train observer's position, s/he observes that point A’/A was struck first by the lightning.
