Which of the following statements is not always true?
a. in a rhombus, the diagonals bisect opposite angles.
b. in a rhombus, the diagonals are perpendicular.
c. in a rhombus, the diagonals are congruent.
d. in a rhombus, all four sides are congruent.

Option C

Step-by-step explanation:

Option A: In a rhombus, the diagonals bisect opposite angles.

It is always true.

Option B : In a rhombus, the diagonals are perpendicular.

It is always True.

Option C : . In a rhombus, the diagonals are congruent.

It is not always true. If the diagonals become congruent then It becomes square.

Option D:  In a rhombus, all four sides are congruent.

It is always True.

Hence Option C is the statement which is not always true.