**Core Concept**
The fossa ovalis is a depression in the right atrium of the heart, formed during embryonic development. It is a remnant of the foramen ovale, a shunt that allowed blood to bypass the lungs in the fetal circulation.

**Why the Correct Answer is Right**
The fossa ovalis is formed when the foramen ovale closes at the end of the third trimester. This closure occurs due to increased pressure in the left atrium and decreased pressure in the right atrium, causing the flap valve of the foramen ovale to seal. The area where the foramen ovale was located becomes the fossa ovalis. The fossa ovalis is a significant anatomical landmark, as it can be used to identify the location of the foramen ovale in the fetal circulation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the fossa ovalis is not formed by the septum secundum. The septum secundum is a separate structure that forms the roof of the foramen ovale, but it does not contribute to the formation of the fossa ovalis.

**Option B:** This option is incorrect because the fossa ovalis is not a remnant of the foramen ovale's flap valve. The flap valve is a critical component of the foramen ovale's closure mechanism, but it does not contribute to the formation of the fossa ovalis.

**Option C:** This option is incorrect because the fossa ovalis is not formed by the septum primum. The septum primum is a critical structure in the development of the heart, but it does not contribute to the formation of the fossa ovalis.

**Clinical Pearl / High-Yield Fact**
The fossa ovalis is a significant anatomical landmark that can be used to identify the location of the foramen ovale in the fetal circulation. It is also a critical structure in the diagnosis of atrial septal defects, which can occur when the fossa ovalis fails to close properly.

**Correct Answer:** C.