B. When realistic, they contribute to a relationship's success
Explanation:
Expectations can only get you in trouble when they differ too much from reality. Specially because they can change widely from situation to situation. Although, once the expectations are close to reality and you're ok with it and can work your way around it, expectations might remind you that you decided to make it work and go with it in times of problem and can remind you that the good sides are happening as expected in times of happiness.
