The crux of the point is 'goal' and 'decision taken by restorers'.

Goal - Uncover Michelangelo's original work - Aspect A

Descion - To keep intact additions that were painted by da Volterra - Aspect B

These 2 aspects try to pull objective in different directions .

The question stem asks what best reconciles Aspect A and Aspect B, so answer got to be something which would try to keep both the aspects in place to the best extent possible.

Choice A tells that da Volterra already had uncovered other additions and applied his own . So when you go on uncovering various layers of additions , the addition of da Volterra would be the last as he is already told in this choice to have uncovered other additions. If da Volterra would not have uncovered other layers of paintings/additions , then in orer to get close to original work of Michelangelo , you would be discarding the addition of da Volterra .This would hurt the decision made .

But choice A , you are at the best reconciled state where goal and decison are met to the best possible extent.
Its just like you have various layers of pianting on original work and the closer the original painting is to the painting which you want to keep intact , the best you can reconcile conflicting aspects