**Core Concept**
Leukoplakia is a condition characterized by the development of white patches or plaques on mucosal surfaces, typically in the oral cavity. The appearance of these lesions is due to the accumulation of dead cells and the alteration of the normal epithelial structure.

**Why the Correct Answer is Right**
The correct answer, A. Cornified layer which imbibes water, is based on the histopathological features of leukoplakia. The cornified layer of the epithelium becomes hyperplastic and imbibes water, leading to the characteristic white appearance of leukoplakia. This is due to the increased number of dead cells and the altered keratinization process in the epithelium. The imbibed water gives the lesion a white, opaque appearance.

**Why Each Wrong Option is Incorrect**
**Option B:** Thick epithelium is a feature of some oral lesions, but it is not the primary reason for the white appearance of leukoplakia. While a thickened epithelium may contribute to the lesion's appearance, it is not the sole cause.

**Option C:** Underlying connective tissue is not directly related to the white appearance of leukoplakia. The lesion's coloration is due to changes in the epithelium, not the underlying tissues.

**Option D:** While all the options may contribute to the appearance of leukoplakia, the correct answer is A. Cornified layer which imbibes water, as it is the primary reason for the white coloration.

**Clinical Pearl / High-Yield Fact**
Leukoplakia can be a premalignant lesion, and its appearance can be a warning sign for oral cancer. A thorough examination and biopsy of the lesion are necessary to rule out malignancy.

**✓ Correct Answer: A. Cornified layer which imbibes water.**