**Core Concept**
The onset of puberty is triggered by the hypothalamic-pituitary-gonadal (HPG) axis, which involves the coordinated secretion of hormones to stimulate the development of reproductive organs and secondary sex characteristics. This complex process is regulated by a cascade of hormone signals, including gonadotropin-releasing hormone (GnRH), luteinizing hormone (LH), follicle-stimulating hormone (FSH), estrogen, and testosterone.

**Why the Correct Answer is Right**
During puberty, the hypothalamus secretes GnRH, which stimulates the pituitary gland to release LH and FSH. LH stimulates the Leydig cells in the testes to produce testosterone, while FSH stimulates the Sertoli cells to produce inhibin and promote spermatogenesis. In females, FSH stimulates the ovarian follicles to produce estrogen, which promotes the development of secondary sex characteristics and the maturation of the reproductive system. Estrogen also inhibits the production of FSH, creating a negative feedback loop to regulate the HPG axis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the hormones required during puberty. While growth hormone plays a role in growth and development during puberty, it is not the primary hormone responsible for the development of secondary sex characteristics.
**Option B:** This option is incorrect because insulin is not directly involved in the regulation of the HPG axis during puberty. While insulin-like growth factor-1 (IGF-1) may play a role in growth and development, it is not the primary hormone responsible for the development of secondary sex characteristics.
**Option C:** This option is incorrect because thyroid hormone is not directly involved in the regulation of the HPG axis during puberty. While thyroid hormone may play a role in overall growth and development, it is not the primary hormone responsible for the development of secondary sex characteristics.

**Clinical Pearl / High-Yield Fact**
The HPG axis is a complex feedback loop that involves the coordinated secretion of multiple hormones to regulate the development of reproductive organs and secondary sex characteristics during puberty. Understanding the role of GnRH, LH, FSH, estrogen, and testosterone in this process is crucial for diagnosing and managing disorders of puberty.

**Correct Answer:** D. GnRH, LH, FSH, estrogen, and testosterone.