**Core Concept**
The question is testing the understanding of postoperative complications following cataract surgery, specifically the most common cause of gradual visual decline in the operated eye.

**Why the Correct Answer is Right**
The most common cause of gradual visual decline after cataract surgery is posterior capsular opacification (PCO). PCO occurs due to the opacification of the posterior capsule, which is the thin membrane behind the intraocular lens implant. This opacification is caused by the proliferation of lens epithelial cells and the deposition of collagen and other proteins. The posterior capsule becomes cloudy, leading to a decrease in vision. This condition is also known as after-cataract.

**Why Each Wrong Option is Incorrect**
* **Option A:** Cataract re-formation is not a common cause of gradual visual decline after phacoemulsification surgery, although it can occur in some cases.
* **Option B:** Uveitis is an inflammatory condition that can cause redness, pain, and watering, which are not consistent with the patient's symptoms.
* **Option C:** Retinal detachment is a serious complication of cataract surgery, but it typically presents with sudden vision loss, flashes, and floaters, which are not described in the patient's history.

**Clinical Pearl / High-Yield Fact**
PCO is a common cause of visual decline after cataract surgery, and it can be treated with a YAG laser capsulotomy, which creates a hole in the cloudy posterior capsule to restore clear vision.

**Correct Answer:** C.