Solutions To Mathematics Textbooks/Proofs and Fundamentals/Chapter 4

= Exercise 4.1.1 = (1), (3) and (6) are functions.

= Exercise 4.1.2 = The first one and the fourth one are functions (here I've assumed that "mother" and "father" were meant biologically, and that not everyone has precisely one person they'd call a "best friend").

= Exercise 4.1.3 = (i) and (iv) represent functions.

= Exercise 4.1.4= (2) and (3) properly describe functions.