Answer
(a) square
(b) square
Work Step by Step
(a) A rhombus that has all angles congruent is a square. If all the angles are congruent, then each angle is $90^{\circ}$ since the sum of the four angles is $360^{\circ}$. Also note that a rhombus has four congruent sides.
(b) A rhombus that has congruent diagonals is a square. If the diagonals are congruent then each of the four angles must be $90^{\circ}$. Also note that a rhombus has four congruent sides.
