Step 1: Understanding the Concept
The question asks for the property related to the side lengths of a square.
Step 2: Detailed Explanation
By definition, a square is a quadrilateral with four equal sides and four right angles (90°).
Let's analyze the options:
(A) Long - short: This is incorrect.
(B) Face to Face equal: This means opposite sides are equal. While true for a square, it's also true for a rectangle. It's not the most specific description.
(C) Four arms equal: Assuming "arms" is a synonym for "sides", this statement means all four sides are equal. This is the defining property of a square's side lengths.
Therefore, "Four arms equal" is the best description of the sides of a square.