Question 1 :
A function cannot be defined inside another function
A function cannot be defined inside the another function, but a function can be called inside a another function.
Question 2 :
Functions cannot return more than one value at a time
True, A function cannot return more than one value at a time. because after returning a value the control is given back to calling function.
Question 3 :
If return type for a function is not specified, it defaults to int
True, The default return type for a function is int.
Question 4 :
In C all functions except main() can be called recursively.
Any function including main() can be called recursively.
Question 5 :
Functions can be called either by value or reference
True, A function can be called either call by value or call by reference.
Call by value means c = sub(a, b); here value of a and b are passed.
Call by reference means c = sub(&a, &b); here address of a and b are passed.