Final answer:
Rhombuses are parallelograms with all sides equal in length, which is their defining characteristic.
Step-by-step explanation:
What all rhombuses have in common is c) 'All sides are equal in length'.
Each rhombus is a type of parallelogram with a few specific characteristics:
- Opposite sides are parallel, which is a characteristic common to all parallelograms and not just rhombuses. This is not unique to rhombuses, so while it's true, it is not the defining characteristic we're looking for.
- While all angles within a single rhombus are congruent to each other, it's not necessary that they are right angles. Some rhombuses do have right angles, but this is not a requirement for all rhombuses.
- All sides of a rhombus are equal in length; this is a key feature that defines a rhombus and distinguishes it from other parallelograms.
- The diagonals of a rhombus are always perpendicular (they intersect at 90° angles), but this is not the only trait they share, so it's not the best answer here.
Therefore, the common trait amongst all rhombuses that best fits from the provided options is that all sides are equal in length.