**Core Concept**
The fossa ovalis is a depression in the interatrial septum, marking the location of the foramen ovale, a communication between the two atria that closes after birth. Understanding the anatomy of the interatrial septum is crucial for identifying the floor of the fossa ovalis.

**Why the Correct Answer is Right**
The floor of the fossa ovalis is formed by the **endocardial cushion** or the **endocardial septum**, which is a fibrous structure that separates the right and left atria. This area is rich in fibrous tissue and is the site where the foramen ovale closes after birth, forming the **fossa ovalis**. The endocardial cushion is located at the junction of the superior and inferior parts of the interatrial septum.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the **limbus fossae ovalis** is a ridge that forms the margins of the fossa ovalis, not its floor. The limbus is a raised structure that borders the fossa ovalis.
**Option B:** This option is incorrect because the **septum primum** is a membrane that forms part of the interatrial septum, but it does not form the floor of the fossa ovalis. The septum primum is a thin membrane that separates the right and left atria.
**Option C:** This option is incorrect because the **septum secundum** is a membrane that forms part of the interatrial septum, but it does not form the floor of the fossa ovalis. The septum secundum is a thicker membrane that forms the posterior part of the interatrial septum.

**Clinical Pearl / High-Yield Fact**
The fossa ovalis is a key landmark in the interatrial septum, and its floor is formed by the endocardial cushion. Identifying the fossa ovalis is crucial in understanding the anatomy of the heart and the pathophysiology of atrial septal defects.

**Correct Answer: C. Septum secundum**