**Core Concept**
The tear film is a complex mixture of lipids, mucins, and water, playing a crucial role in maintaining the health of the cornea. The tear film consists of three distinct layers: the lipid layer, the aqueous layer, and the mucous layer. Each layer serves a unique function, and their composition and thickness are essential for maintaining proper eye function.

**Why the Correct Answer is Right**
The thickest layer of the tear film is the aqueous layer, which constitutes approximately 98% of the tear film's volume. This layer is primarily composed of water (98%) and electrolytes, with a small percentage of proteins, mucins, and lipids. The aqueous layer serves as a reservoir for nutrients and waste products, helping to maintain the cornea's health and hydration. The high water content of this layer also helps to regulate the pH and osmolality of the ocular surface.

**Why Each Wrong Option is Incorrect**
**Option A:** The lipid layer is the thinnest and most superficial layer of the tear film, playing a crucial role in reducing evaporation and maintaining corneal health. However, it is not the thickest layer.
**Option B:** The mucous layer is the most superficial layer of the tear film, produced by the goblet cells of the conjunctiva. It helps to spread the tears evenly over the ocular surface and facilitates the movement of tears. While it is essential for tear film function, it is not the thickest layer.
**Option C:** This option is not provided.

**Clinical Pearl / High-Yield Fact**
The tear film's composition and thickness can be affected by various factors, including age, climate, and certain medical conditions. For example, dry eye syndrome is characterized by a reduction in tear film volume and an imbalance in tear film composition, leading to eye irritation and discomfort.

**Correct Answer: C. Aqueous layer. The aqueous layer of the tear film is the thickest layer.