**Core Concept**
The underlying principle being tested is the understanding of **glaucomatous field defects** and their progression in **open-angle glaucoma**. This condition is characterized by a gradual loss of vision due to damage to the **optic nerve**, often associated with increased **intraocular pressure**.

**Why the Correct Answer is Right**
Since the correct answer is not provided, let's discuss the general approach to this question. In open-angle glaucoma, the earliest field defect often occurs in the peripheral field, specifically in the **nasal step** or **arcuate field**. However, without the correct answer choice, we can't specify which option is correct. Normally, the earliest defects are subtle and can be detected through meticulous **visual field examination**.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific answer choices, it's challenging to address each option directly. However, incorrect options might include field defects that are not typically the earliest signs, such as central or temporal field defects.
**Option B:** Similarly, without the correct context, we can't accurately describe why this option might be incorrect.
**Option C:** and **Option D:** would follow the same reasoning, focusing on the typical progression of glaucomatous field defects.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **open-angle glaucoma** often presents with no symptoms until significant vision loss has occurred, making regular **ocular examinations** crucial for early detection. The **nasal step** and **arcuate scotomas** are classic early field defects.

**Correct Answer:** Correct Answer: C. Nasal step.