A

A unit less quantity is fundamentally a dimensionless quantity, meaning it does not possess any physical dimensions such as length, mass, or time.
Step 1: In physics, we categorize quantities based on their dimensional characteristics. A dimensionless quantity is one that can be expressed without any reference to a physical dimension.
Step 2: Common examples include ratios, percentages, and coefficients (like the coefficient of friction, which is a ratio of two forces).
Step 3: Therefore, because they are defined without dimensions, unit less quantities never have a non-zero dimension.
Hence, the correct answer is A.