Concept:
A vector quantity possesses both magnitude and direction. The magnitude of a vector represents its size or numerical value and is always non-negative. In general, vector magnitude is considered a positive quantity.
For example:
\[
|\vec{A}| > 0
\]
Step 1: Understand the meaning of vector magnitude.
Magnitude refers to the size or amount of a vector quantity.
Step 2: Determine the nature of magnitude.
Since magnitude represents size, it cannot be negative.
Therefore, the magnitude of a vector is considered:
\[
\text{Positive quantity}
\]
Hence, the correct answer is:
\[
\boxed{(A)\ \text{Positive quantity}}
\]