Final answer:
Delay of gratification during the preschool years is related to academic achievement, social competence, and assertiveness, but not lower stress levels.
Step-by-step explanation:
Research by Walter Mischel has shown that delay of gratification during the preschool years is related to academic achievement, social competence, and assertiveness. However, it is not related to lower stress levels.
In Mischel's marshmallow study, children who were able to delay gratification and wait for a larger reward demonstrated greater success in high school, including higher SAT scores, positive peer relationships, and lower likelihood of substance abuse issues. Therefore, the correct answer is D) Lower stress levels.