**Core Concept**
The muscles responsible for foot inversion are a group of intrinsic muscles in the foot, which include the tibialis posterior, flexor digitorum longus, and the peroneus longus and tibialis anterior when considering the broader musculature of the lower limb. Inversion of the foot is a crucial component of balance and posture, as it enables the foot to adjust to uneven surfaces and maintain equilibrium.

**Why the Correct Answer is Right**
Inversion of the foot is primarily facilitated by the tibialis posterior muscle, which has a broad origin on the tibia, fibula, and talus, and inserts into the cuneiform and cuboid bones of the foot. This muscle is responsible for the majority of the foot's inversion due to its long tendon, which allows it to exert a strong pulling force on the medial aspect of the foot. The peroneus longus muscle, however, is primarily responsible for eversion of the foot, not inversion.

**Why Each Wrong Option is Incorrect**
* **Option A:** The tibialis posterior muscle is indeed responsible for foot inversion, making this option incorrect.
* **Option B:** The flexor digitorum longus muscle does contribute to foot inversion, particularly in the plantar flexion of the toes, but it is not the primary muscle responsible for inversion.
* **Option D:** The peroneus longus muscle is primarily responsible for eversion of the foot, not inversion.

**Clinical Pearl / High-Yield Fact**
The peroneal muscles (peroneus longus and peroneus brevis) are often injured in ankle sprains, leading to chronic instability and pain. This is a classic exam trap, as the peroneal muscles are often overlooked in favor of the tibialis posterior muscle when considering foot inversion.

**Correct Answer: C. Peroneus longus**