**Core Concept**
The question tests the student's knowledge of the anatomy of the foot, specifically the bones and their associated muscles. In the foot, each bone has a unique set of muscles that attach to it, allowing for movement and stability. Understanding the anatomy of the foot is crucial for diagnosing and treating various foot-related conditions.

**Why the Correct Answer is Right**
The calcaneus, also known as the heel bone, is not attached by any muscle. This is because the calcaneus is a fixed bone that serves as a base for the foot, providing stability and support. Unlike other bones in the foot, the calcaneus does not have any muscles that directly attach to it. Instead, it is surrounded by tendons and ligaments that provide support and stability.

**Why Each Wrong Option is Incorrect**
* **Option A:** The talus bone is attached by muscles such as the flexor hallucis longus, which runs along the medial aspect of the talus.
* **Option B:** The cuboid bone is attached by muscles such as the peroneus longus, which runs along the lateral aspect of the cuboid.
* **Option C:** The navicular bone is attached by muscles such as the tibialis posterior, which runs along the medial aspect of the navicular.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the calcaneus is a fixed bone, and any abnormal movement or instability in the foot can be related to the surrounding soft tissues, such as tendons and ligaments.

**Correct Answer: D. Calcaneus**