________ is a crystalline form of carbon that has layered structure of hexagonal rings arranged parallel to each other.

Explanation

The answer to the question is **Graphite**.

### Step-by-Step Solution:

1. **Understanding Allotropes of Carbon**:
- Carbon exists in different forms known as allotropes. The most common allotropes include diamond, graphite, and fullerenes (like buckminsterfullerene).

2. **Identifying the Structure**:
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