Final answer:
The statement that is always true about rhombuses is that all rhombuses have equal side lengths. Their diagonals are also perpendicular, while not all rhombuses have congruent angles or right angles.
Step-by-step explanation:
The statement about rhombuses that is always true is C) All rhombuses have equal side lengths. Rhombuses are a special type of polygon known specifically for having all sides of equal length. Although not all rhombuses have congruent angles, right angles, or are squares, every rhombus does indeed have sides of equal length, making this a defining characteristic of the shape. Option B is also always true for rhombuses as their diagonals are perpendicular to each other, which is a result of the shape's properties where the diagonals bisect each other at right angles. Therefore, options A and D are not always true, as the angles in a rhombus need not be congruent, and a rhombus does not have to have a right angle unless it is a square (which is a special type of rhombus).