A rhombus is a quadrilateral with certain unique properties. The correct answer is option 'D', which states that a rhombus is a quadrilateral in which the diagonals bisect opposite angles. Let's understand why this is the correct answer.

Diagonals of a Rhombus:
- A rhombus has two pairs of opposite sides that are equal in length.
- The diagonals of a rhombus are the line segments connecting opposite vertices.

Bisecting Opposite Angles:
- When we say that the diagonals of a rhombus bisect opposite angles, it means that the diagonals divide the rhombus into four congruent triangles.
- Each diagonal cuts the rhombus into two equal parts, and these parts are congruent.
- As a result, the opposite angles formed by the diagonals are equal in measure.

Proof of Diagonal Bisecting Opposite Angles:
To prove that the diagonals of a rhombus bisect opposite angles, we can use the concept of congruent triangles.
Let's consider a rhombus ABCD with diagonals AC and BD.

Proof:
1. In rhombus ABCD, opposite sides are equal. Therefore, AB = BC = CD = DA.
2. Diagonals of a rhombus bisect each other. Therefore, AC and BD intersect at point O such that AO = OC and BO = OD.
3. Now, consider triangle ABO and triangle BCO.
- AB = BC (opposite sides of the rhombus)
- AO = OC (diagonals bisect each other)
- BO = BO (common side)
By side-side-side congruence, we can conclude that triangle ABO is congruent to triangle BCO.
4. Congruent triangles have equal corresponding angles. Therefore, angle AOB = angle BOC.
5. Similarly, we can prove that angle COD = angle DOA.
6. Hence, the diagonals of a rhombus bisect opposite angles.

Conclusion:
Based on the properties of a rhombus and the proof provided, it is clear that option 'D' is the correct answer. A rhombus is a quadrilateral in which the diagonals bisect opposite angles.