**Core Concept**
In the context of dental restorations, a "better restorative option" refers to a treatment approach that aims to preserve as much of the natural tooth structure as possible, while also ensuring the tooth's functionality and longevity. This concept is crucial in managing grossly carious teeth, where the tooth's structure is severely compromised due to extensive decay.

**Why the Correct Answer is Right**
The correct answer is **inlay**. An inlay is a type of dental restoration that is fabricated outside the mouth and then cemented into place within the prepared tooth. This option is considered better than the others because it allows for the removal of carious dentin and the placement of a strong, durable restoration that can withstand occlusal forces. Inlays are particularly useful in grossly carious teeth as they can be made from a variety of materials, including ceramic, composite resin, or metal, and can be designed to match the color and texture of the surrounding tooth structure.

**Why Each Wrong Option is Incorrect**
**Option A:** **Onlay**. While an onlay is a type of dental restoration that can be used to treat grossly carious teeth, it is typically larger than an inlay and covers more of the tooth surface. This makes it a less conservative option compared to an inlay.

**Option B:** **Full-coverage crown**. A full-coverage crown is a more invasive option that covers the entire tooth, including the occlusal surface. While it can be used to treat grossly carious teeth, it is generally considered a more destructive option than an inlay.

**Option C:** **Root canal therapy**. Root canal therapy is a treatment option for teeth with infected or damaged pulp tissue, but it is not a restorative option for grossly carious teeth. While root canal therapy may be necessary in conjunction with an inlay or other restoration, it is not a suitable option for addressing the structural compromise caused by caries.

**Clinical Pearl / High-Yield Fact**
When considering restorative options for grossly carious teeth, it is essential to weigh the benefits of each approach in terms of preservation of natural tooth structure, durability, and aesthetic outcomes. In general, more conservative approaches, such as inlays or onlays, are preferred whenever possible.

**Correct Answer:** C. Onlay.