Good question. This is from the OG.
Herez the OG explanation.Will be helpful to others
It may help to imagine a simplified version of the sentence and substitute the other answer choices for "The price increase was what brought..."
Bringing suggests an ongoing condition and is incompatible with an action that was completed shortly after the Second World War.
Similarly, has brought indicates action that continues up to the present; the past tense brought is needed to parallel was.
510 on my first GMAT. 610 on second GMAT.!
The struggle continues.