The answer is D. The figure obtained when you join the midpoint of adjacent sides of a rectangle of side 8 cm and 6 cm is a rhombus with an area of 24 cm squared. A rhombus is a parallelagram with 4 equal sides, 2 opposite equal obtuse angles and 2 opposite equal acute angles. A square is also a rhombus.
The area is calculated using a mathematical formula: 1/2 x the product of the lengths of the diagonal sides. Another way to categorize a rhombus is to say it is an equilateral quadralateral, which just means that the shape has 4 sides and all 4 of them are equal.
The figure, which is obtained after you join the midpoint of adjacent sides of a rectangle having the side length 8 cm and with 6 cm, is a rhombus of area 24 cm square. Hence, option D is the right option. The study of shapes is usually referred to as Geometry. Geometry is a vast field however which includes a lot of other sub-fields.
Nevertheless, the analytical, qualitative, and quantitative study of shapes and their properties are covered by geometry. Many people find it as quite an interesting area of maths due to the mesmerizing shapes and their interesting properties.