..... is the basic repeated structural unit of a crystalline solid.

Options

A. Monomer
B. Molecule
C. Unit cell
D. Atom

Answer

The basic repeated structural unit of a crystalline solid is known as the **unit cell**.

### Step-by-Step Solution:

1. **Understanding Crystalline Solids**: Crystalline solids are materials whose constituents (atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions.

2. **Defining the Unit Cell**: The unit cell is the smallest repeating unit of a crystal lattice. It defines the structure of the entire crystal. Just like cells are the basic building blocks of living organisms, unit cells are the fundamental building blocks of crystalline solids.

3. **Visualizing the Unit Cell**: Imagine a three-dimensional lattice structure. Each unit cell can be visualized as a small box that, when repeated in three dimensions, creates the entire crystal structure.

4. **Identifying the Correct Term**: In the context of crystalline solids, the term "unit cell" is specifically used to describe this basic repeating unit. Other terms like "monomer" refer to repeating units in polymers, while "atom" and "molecule" refer to smaller components of matter.

5. **Conclusion**: Therefore, the answer to the question is that the basic repeated structural unit of a crystalline solid is the **unit cell**.