Provide an Appropriate Response $\mathrm { f } _ { \mathrm { X } }$

Multiple Choice

Provide an appropriate response.
-For which of the following functions do both $\mathrm { f } _ { \mathrm { X } }$ and $\mathrm { f } _ { \mathrm { y } }$ exist?

A) I. and II.
B) II. and IV.
C) II., III., and IV.
D) IV. only