The following century is usually considered as "Classical period of cell biology"

Options

A. 18th
B. 19th
C. 20th
D. 21st

Answer

To determine which century is considered the "Classical period of cell biology," we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Question**: We need to find out which century is referred to as the "Classical period of cell biology."

2. **Analyze the Options**: The options provided are:
- 18th century
- 19th century
- 20th century
- 21st century

3. **Research Historical Context**: The foundation of cell biology was significantly developed in the 19th century. Notably, in 1839, scientists Theodor Schwann and Matthias Schleiden proposed the cell theory, which is a fundamental concept in cell biology.

4. **Connect the Information**: Since the cell theory was articulated in 1839, it is clear that this event took place in the 19th century. This period is recognized for establishing key principles that form the basis of modern cell biology.

5. **Conclude the Answer**: Based on the historical significance and the development of cell biology, the century that is considered the "Classical period of cell biology" is the 19th century.

### Final Answer:
The 19th century.