**Core Concept**
The question is testing the student's knowledge of the Graafian follicle, a mature follicle in the ovary that contains an oocyte. The follicle is composed of several layers of cells that play crucial roles in oocyte maturation and ovulation. The cells surrounding the oocyst are a critical component of the follicular structure.

**Why the Correct Answer is Right**
The correct answer is the layer of cells that directly surrounds the oocyst, providing essential support and nourishment to the developing oocyte. This layer is composed of **granulosa cells**, which secrete hormones like estrogen and progesterone that regulate the menstrual cycle and prepare the uterus for implantation. The granulosa cells also play a key role in oocyte maturation and ovulation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specifically refer to the cells surrounding the oocyst. While the theca interna is a layer of cells in the Graafian follicle, it is not the layer that directly surrounds the oocyst.
**Option B:** This option is incorrect because it is not a specific layer of cells in the Graafian follicle. The oocyte itself is not surrounded by a layer of cells, but rather is contained within the follicle.
**Option C:** This option is incorrect because it is not a recognized layer of cells in the Graafian follicle. The cumulus cells are a type of granulosa cell that surrounds the oocyst during ovulation, but they are not the same as the layer of cells that surrounds the oocyst in the Graafian follicle.

**Clinical Pearl / High-Yield Fact**
The Graafian follicle is a critical structure in the menstrual cycle, and the cells surrounding the oocyst play a key role in oocyte maturation and ovulation. Understanding the structure and function of the Graafian follicle is essential for diagnosing and managing reproductive disorders.

**Correct Answer: C. Granulosa cells**