.............. linked with a surface area is defined as the number of magnetic field lines passing through that area perpendicularly.

Explanation

To solve the question, we need to identify the quantity that is defined as the number of magnetic field lines passing through a surface area perpendicularly. This quantity is known as magnetic flux.

### Step-by-Step Solution:

1. **Understanding the Concept**: Magnetic flux is a measure of the quantity of magnetism, taking into account the strength and the extent of a magnetic field. It quantifies the number of magnetic field lines that pass through a given surface area.

2. **Defining Magnetic Flux**: The magnetic flux (Φ) through a surface is defined mathematically as:
\[
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