This is not correct:

printf("%d", "Enter a number and I will tell you the absolute value: ", a);

The first argument to printf is a format string, and the following arguments are values that fulfill that format string. Your format string is "%d" which means you intend to print an int, but the next argument is a string.

Since you just want to print a string, make that the format:

printf("Enter a number and I will tell you the absolute value: ");

This is also a problem:

printf("My absolute value of %d is %lf\n", a, absolute(a));

Because the %lf format specifier expects a double but absolute returns an int. Because of the variardic nature of printf, the int is not implicitly converted to a double, so you have a format string argument mismatch. You should use %d instead:

printf("My absolute value of %d is %d\n", a, absolute(a));