The mango fruit is called a stone fruit because it has:
A.Skinny Epicarp
B.Stony Mesocarp
C.Fleshy endocarp
D.Hard Endocarp

Hint: Mango is a fleshy fruit which comes under the subdivision of a stone fruit. The outer layer of a fleshy fruit is known as the pericarp. The pericarp is further made up of Epicarp the outermost one, the mesocarp the middle one and the endocarp the innermost layer.

Complete answer:
A mango is a type of tropical tree belonging to the flowering plant genus Mangifera. It is a stone fruit called as Drupe, which is produced from the mature ovary of one or more flowers. A drupe is a type of fleshy fruit which has thin skin, a seed in the centre and with a hard covering over it. In drupes, the pericarp forms the edible tissue around the seeds. Pericarp is the part of the flower which is edible in fruits such as citrus and stone fruit.
Epicarp is the outermost layer of the pericarp which forms the tough outer skin of the fruit. The middle layer of the fleshy pericarp is called the mesocarp which is found between the epicarp and the endocarp. It is usually the part of the fruit that is eaten. The most innermost layer of the fleshy fruit is endocarp, which directly surrounds the seeds. Endocarp layer can be either membranous or thick and hard depending upon the type of fruit it is. Endocarp is the innermost part of the flower which is eaten in membranous fruits like citrus or it can be either a hard seed in stone fruits like mango.
These hardened stones contain edible seed. A Mango is a drupe which contains thin epicarp, indehiscent mesocarp and a hardened endocarp. In the mango the inner layer which is hard and stony is known as endocarp and the presence of this layer makes mango a stony fruit.

Hence, the correct answer is option (D)

Note: Fruit is the ripened form of an ovary. It is the protective covering for the seed and the embryo. The epicarp and mesocarp together make up the peel in the citrus fruit. The mesocarp is the mid layer between the exocarp and the endocarp. In a plant like mango, the fleshy part of the fruit which is eaten between the skin and the seed is called Mesocarp.