**Core Concept**
Thymosin is a group of peptide hormones produced by the thymus, playing a crucial role in the development and maturation of T-lymphocytes (T cells). The thymus is a vital organ in the immune system, responsible for the maturation and selection of T cells, which are essential for cell-mediated immunity.

**Why the Correct Answer is Right**
Thymosin, specifically thymosin beta 4 (Tβ4), is a peptide hormone that promotes the maturation of pre-T cells in the thymus. This process is essential for the development of a functional immune system. During thymocyte maturation, thymosin beta 4 regulates the expression of various genes involved in T cell development, including the rearrangement of the T cell receptor (TCR) genes. Thymosin beta 4 also plays a role in the negative selection of autoreactive T cells, preventing autoimmune responses. The maturation of pre-T cells is a complex process that requires the coordinated action of multiple hormones, growth factors, and cytokines, including thymosin beta 4.

**Why Each Wrong Option is Incorrect**
**Option A:** This statement is incorrect as thymosin is not a hormone produced by the pituitary gland, but rather by the thymus itself. The pituitary gland produces hormones that regulate various bodily functions, including growth, metabolism, and reproductive processes, but it is not involved in the production of thymosin.
**Option B:** This statement is incorrect as thymosin does not confer passive immunity to non-sensitized hosts. Passive immunity is conferred through the transfer of antibodies from one individual to another, whereas thymosin plays a role in the development and maturation of T cells, which is an active process.
**Option C:** This statement is incorrect as antiserum against thymocytes is not a characteristic of thymosin. Antiserum is a type of antibody-based treatment used to neutralize toxins or diseases, but it is not related to thymosin or its function.

**Clinical Pearl / High-Yield Fact**
Thymosin beta 4 is not only involved in T cell development but also has roles in wound healing, tissue regeneration, and protection against infection. Its potential therapeutic applications make it an area of active research in immunology and regenerative medicine.

**✓ Correct Answer: D. A thymic hormone that induces maturation of pre-T cells**