Step 1: Understanding properties of a square
A square is a regular quadrilateral. It has:
- Four sides
- Four right angles ($90^\circ$ each)
- Opposite sides parallel
Step 2: Key property of a square
The most important property is that **all four sides of a square are equal in length.**
This makes it different from rectangles, where only opposite sides are equal.
Step 3: Analyzing options
(A) Big and small $\Rightarrow$ Incorrect, squares do not have unequal sides
(B) Four sides are equal $\Rightarrow$ Correct by definition
(C) Opposite sides are equal $\Rightarrow$ True for rectangles, not the best choice for square
(D) None of these $\Rightarrow$ Incorrect
\[
\boxed{\text{Correct Answer: (B) Four sides are equal}}
\]