7.
float (*abc)(float x);

put in all the parentheses:
float ((*abc)(float x));

  1. abc is being declared (not x),
  2. it is a pointer to something:
  3. a function that takes a float and returns ...
  4. another float.
abc is not a function here. Abc is a normal variable, that can contain a pointer to a function. The function it points to must be one that takes a float argument and returns a float result, like the mathematical function sine for example.

You could in fact use the variable for exactly that purpose, by writing the assignment abc=sin;. Normally you would expect to have to put an & in front of the name sin to produce a pointer, but just like array names, function names are automatically assumed represent pointers. Most compilers will whine bitterly if you put the & in.

After such an assignment, you could say x=abc(y);, which would have exactly the same effect as x=sin(y);, except that now you can have a program that dynamically chooses which function to call.

NOTE You may think that I left something out in the last paragraph. abc isn't a function, its a pointer to a function, so we should say (*abc)(y) instead of abc(y). Unfortunately, C doesn't look at it that way. The name of a function always stands for the address of that function, just like with an array. The name sin actually represents a pointer to the sine function; it is as though an & had been automatically added. So when you say sin(y) you are really putting a function pointer before the opening parenthesis, not a real function at all. C wouldn't know how to handle a real function if you gave it one. So, abc, being declared as a function pointer is of the same status as sin; no * is needed or even allowed.
        Normal function names, even though they are pointers, are still constants. You can't assign a new value to sin. When you define a function like this: 
        void fff(char x)
        { printf("%c%c%c",x); }
it is as though you really defined a constant pointer-"variable" of the right type, and somehow initialised it with the right address, like this:

                void (* const f)(char) = correct memory address;

Yes, that is proper C. The const before the name f specifies that f is a constant, which shouldn't surprise you. Apart from that it has the exact same form as the declaration of a function-pointer variable, which is what it is.

abc is just a simple pointer, so its size is 4.