Function is an assignation that for each element of first set assigns one and only one element from second set.
1) NO {element 2 from set one has assigned two elements (b, c) from set two}
2) YES
3) YES {several elements from set one can have assigned the same element from set two}
4), 5) YES
6) NO {for x from about -4 to about 0.5 there are two y}