Introduction
Studies in human development focus on understanding the changes that occur in individuals from conception to death. This field of study examines physical, cognitive, emotional, and social development throughout the lifespan. The correct answer to this question is option 'D', which includes the period from conception to death. Let's explore this answer in detail.

Conception and Prenatal Development
The period of human development begins at conception, which is the moment when an egg is fertilized by a sperm. Conception marks the beginning of prenatal development, which encompasses the growth and changes that occur in the developing embryo and fetus. During this stage, the embryo gradually develops organs, limbs, and a nervous system, while the fetus continues to grow and mature.

Infancy and Early Childhood
After birth, the period of infancy and early childhood begins. This stage of human development involves rapid physical growth, as well as the development of basic motor skills, language, and social interactions. Infants and young children undergo significant cognitive and emotional development during this time, as they learn to communicate, understand their environment, and form attachments with caregivers.

Middle Childhood and Adolescence
The next phase of human development includes middle childhood and adolescence. During middle childhood, children continue to refine their cognitive abilities, develop friendships, and gain independence. In adolescence, individuals experience significant physical, cognitive, and emotional changes as they transition from childhood to adulthood. This period is characterized by rapid growth, the onset of puberty, and the development of identity and autonomy.

Adulthood and Aging
The later stages of human development include adulthood and aging. Adulthood is marked by further cognitive and emotional development, as individuals establish careers, form intimate relationships, and navigate various life transitions. As individuals age, they undergo physical and cognitive changes associated with the aging process. The study of human development considers the challenges and opportunities associated with different stages of adulthood, including parenting, career development, and retirement.

Conclusion
In summary, studies in human development encompass the entire lifespan, from conception to death. This field of study explores the physical, cognitive, emotional, and social changes that occur throughout different stages of life. Understanding human development is crucial for professionals in fields such as psychology, education, healthcare, and social work, as it provides insight into the factors that shape individuals' lives and experiences.