why character are called Integral type in C programming language?

Because they are represented with non-modifiable integer values.

For ASCII characters, for example, these values range between 0 and 255.

You may perform integer operations with objects of the type char. For example you may decrease or increase a value stored in an object of the type char, or add or subtract a value.

For example if an object of the type char contains a symbol that represents a digit as for example

char c = '5';

then you can get the digit as a number the following way

int digit = c - '0';

The variable digit will contain the number 5.

Pay attention to that the type char can behave either as the type signed char or unsigned char dependent on compiler options.

In C opposite to C++ integer character constants have the type int .