A parallelogram in which two adjacent sides are equal is a ______________.
(a).Square
(b). Rhombus
(c). Trapezium
(d). Kite

HINT:-Before solving this question, let us know about polygons, parallelogram, square, rhombus, trapezium and kite.
POLYGONS: A polygon is any 2 – dimensional shape formed with straight lines. For example: Triangles, quadrilaterals, pentagons, hexagons, etc. are all examples of polygons.
PARALLELOGRAM: A parallelogram is a four – sided shape that is made up of two pairs of straight parallel lines that are equal in length.
RHOMBUS: A parallelogram in which all the sides are equal is called a rhombus. The length of all of its sides is equal.
RECTANGLE: A parallelogram in which all the angles are right angles and the length of the diagonals is equal is called a rectangle.
SQUARE: A parallelogram with all the sides of equal length is called a square. All of its angles are equal and their measure is 90° each.
KITE: A kite is a quadrilateral that has equal adjacent sides. In contrast, a parallelogram also has two pairs of equal-length sides, but they are opposite to each other rather than adjacent. So, a kite is not a parallelogram.

Complete step-by-step solution -
Hence, according to the hint provided above, option (b) Rhombus is the correct option.
A rhombus is a parallelogram in which the adjacent sides are equal.
Note:-Let us learn about the conditions in a square and rhombus.
A rhombus is a quadrilateral with all sides equal in length. A square is a quadrilateral with all sides equal in length and all interior angles as right angles. Thus a rhombus is not a square unless the angles are all right angles. A square however is a rhombus since all four of its sides are of the same length.