**Core Concept**
A removable partial denture (RPD) is a dental appliance used to replace missing teeth. Indirect retainers are a type of RPD attachment that helps stabilize the denture by distributing forces to the supporting tissues and preventing unwanted movement or rotation. The need for an indirect retainer depends on the design and placement of the RPD.

**Why the Correct Answer is Right**
An indirect retainer is required in RPDs with posterior extensions, particularly those with bilateral posterior extensions, to prevent the denture from tilting or rotating in the posterior region. This is because the posterior extensions can create a fulcrum effect, leading to unwanted movement of the denture. The indirect retainer helps to neutralize this effect by providing additional support and stability to the denture. This is especially true for RPDs with bilateral posterior extensions, where the forces acting on the denture are more pronounced.

**Why Each Wrong Option is Incorrect**
**Option A:** A unilateral RPD with anterior extensions may not require an indirect retainer, as the forces acting on the denture are minimal and the anterior region provides sufficient support.
**Option B:** A RPD with no posterior extensions may not require an indirect retainer, as the denture is less likely to experience unwanted movement or rotation.
**Option C:** A RPD with a clasp assembly in the anterior region may not require an indirect retainer, as the clasp assembly provides sufficient retention and stability to the denture.

**Clinical Pearl / High-Yield Fact**
When designing an RPD, it's essential to consider the type of retention and stability needed, particularly in cases with posterior extensions. An indirect retainer can help prevent unwanted movement or rotation of the denture, ensuring a more stable and functional outcome for the patient.

**Correct Answer:** C. A RPD with posterior extensions, particularly bilateral posterior extensions, definitely requires an indirect retainer to prevent unwanted movement or rotation.