Which of the following is a primary sex characteristic that changes at puberty? a. A growth spurt in height, especially for boys. b. Development of breasts for girls. c. Full development of external genitalia in both sexes. d. Facial hair and deepened voice for boys. e. Appearance of pubic and underarm hair in both sexes.

Step 1: Define the Problem

Identify the primary sex characteristic that changes at puberty from the given options.

Step 2: Break Down the Problem

• Understand what primary sex characteristics are: These are directly related to the reproductive organs and their development.
• Review each option to determine if it describes a primary sex characteristic.

Step 3: Apply Relevant Concepts

• Primary sex characteristics involve the maturation of the reproductive organs.
• Secondary sex characteristics include changes like growth spurts, breast development, facial hair, voice changes, and body hair.

Step 4: Analysis, Verify and Summarize

• Option a: Growth spurt is a secondary characteristic.
• Option b: Breast development is a secondary characteristic.
• Option c: Full development of external genitalia is a primary characteristic.
• Option d: Facial hair and deepened voice are secondary characteristics.
• Option e: Pubic and underarm hair are secondary characteristics.

Final Answer

c. Full development of external genitalia in both sexes.