Answer
a) Not a function.
b) A function.
c) A one-to-one function.
d) A function.
Work Step by Step
a) Some of the x-values have more than one y-value.
b) Some y-values are repeated.
c) Each x-value has its own y-value, and no y-value is repeated.
d) Almost all y-values are repeated (with the exception of $0$)
