**Core Concept**
Gene therapy is a medical treatment that uses genes to prevent or treat diseases. It involves modifying an individual's genes to correct genetic disorders or to enhance the body's natural defenses against diseases. Gene therapy can be delivered through various methods, including viral vectors, plasmids, and RNA-based approaches.

**Why the Correct Answer is Right**
Gene therapy methods include viral vectors, which are engineered viruses that are designed to deliver genetic material to specific cells in the body. Viral vectors can be derived from adenoviruses, adeno-associated viruses, lentiviruses, or retroviruses, each with its own strengths and limitations. These vectors can be used to deliver therapeutic genes to various tissues, including muscle, liver, and immune cells. The choice of viral vector depends on the specific disease being treated, the target tissue, and the desired duration of gene expression.

**Why Each Wrong Option is Incorrect**
**Option A:** Gene editing methods, such as CRISPR-Cas9, are not considered gene therapy methods, as they involve making precise changes to the genome rather than introducing new genes.
**Option B:** RNA-based approaches, such as RNA interference (RNAi), are used to silence specific genes rather than introduce new ones, and are therefore not considered gene therapy methods.
**Option C:** Plasmids are small, self-replicating circular DNA molecules that can be used to deliver genetic material, but are not considered gene therapy methods on their own, as they require a delivery system to reach the target cells.

**Clinical Pearl / High-Yield Fact**
Gene therapy has shown promise in treating inherited diseases, such as severe combined immunodeficiency (SCID), and has also been explored for use in cancer therapy. However, gene therapy carries risks, including insertional mutagenesis, immune responses to the vector, and off-target effects, highlighting the need for careful consideration of the benefits and risks before using these methods.

**Correct Answer: C. Plasmids are small, self-replicating circular DNA molecules that can be used to deliver genetic material.**