In a function, each input produces a single output, but different inputs moy produce the same output

Ex$.5$ Which of the following rules describe functions?

a$)$ Use the optical reader at the checkout counter of the grocery store to convert codes to prices.

b$)$ A loan officer at Bank of America aus each applicant to provide his or her Social Security number.

c$)$ A loan officer at Bank of America asks each applicant to provide his or her credit card number.

d$)$ Enter a number in a calculater and press the $x^2$ key.