Our sentence selection must have the following features: it includes a transition phrase and has the effect of concluding or summarizing.
A) is the best answer of the options. "In the end" is a transition phrase that has the effect of wrapping up the ideas discussed, similar to "in conclusion". The conclusion, following what we can assume is a description of events that explain Peter's decision, sums up that his motives were clear and establishes an end to the explanation.
