The most appropriate option to fill the gap is B. would have stepped down. The sentence is in the modal form, which is used to talk about hypothetical situations in the past. The correct form is “If + past perfect, would + have + past participle”. In this case, the correct sentence would be “If he had considered his public image carefully, he would have stepped down from his office in the election”.

The terms “step down” and “stand down” are interchangeable and can be used to indicate the act of resigning from a position or office. However, “step down” is more commonly used when discussing political positions or offices. In the sentence “Had he considered his public image carefully, he would have stepped down from his office in the election,” the phrase “would have stepped down” is used to indicate that the person would have resigned from their position if they had considered their public image more carefully.