Final answer:
A study of motivation and gender most closely aligns with social psychology, which considers how external factors influence individual thoughts and behaviors. Developmental psychology also considers gender within the context of psychosocial development throughout a person's life.
Step-by-step explanation:
The psychological domain that would include a study of motivation and gender is social psychology. This field focuses on how individual thoughts, feelings, and behaviors are influenced by the actual, imagined, or implied presence of others, including situational factors - which could encapsulate motivation - and attributes pertaining to gender. However, since motivation and gender studies can intersect various domains, developmental psychology also covers aspects of gender as it examines changes in psychosocial development over a person's lifespan.
Cognitive psychology, on the other hand, is primarily concerned with the study of human thinking, such as learning, attention, memory, language, thinking, reasoning, and creativity. It does not focus directly on motivation and gender.
Lastly, epigenetics is the field of study that emphasizes genes and their expression, which is distinct from social and developmental psychology's emphases on environmental influences and individual experiences. It would not typically include a study focused on motivation and gender.