Step 1: Primary follicles are immature ovarian follicles containing an oocyte. Females are born with approximately 1 to 2 million primordial follicles, which significantly decrease with age.
Step 2: By puberty, the number of primary follicles is reduced to about 300,000 to 400,000 due to follicular atresia, a natural process of follicle degeneration.
Step 3: Option (A): Incorrect. The 2 million follicles present at birth decrease before puberty.
Option (B): Incorrect. 1 million follicles is an estimate for early childhood, not puberty.
Option (C): Correct. Thousands (approximately 300,000–400,000) of primary follicles are present during puberty.
Option (D): Incorrect. Primary follicles are present during puberty.
Step 4: Conclusion. During puberty, the number of primary follicles in the ovaries is reduced to thousands due to natural follicular atresia. \[ \therefore \text{The correct answer is: Thousands.} \]