Which of the following statements is true about a rhombus?

A rhombus is defined as a type of polygon that is a quadrilateral with certain specific properties. One of the fundamental characteristics of a rhombus is that all four sides are equal in length, which is what makes option B the correct statement. This property ensures that while the angles can vary, the sides maintain uniformity in length throughout the figure.

The other characteristics of a rhombus include opposite angles being equal and adjacent angles being supplementary, meaning that they add up to 180 degrees. Because of this, angles in a rhombus can be acute or obtuse, but they are not restricted to being acute, which is why stating that the angles are always acute is not true.

The fact that all sides are equal in length allows a rhombus to also be classified as a parallelogram, where the opposite sides are parallel. However, it is not true that a rhombus has only one pair of parallel sides, as both pairs of opposite sides in a rhombus are parallel.

Therefore, while the other statements provide information about a rhombus, it is the equality of all sides in a rhombus that correctly identifies option B as the true statement about this geometric shape.