Which of the Following Functional Forms for Utility Suggests the Greatest

Which of the following functional forms for utility suggests the greatest substitution effect when starting at the point where $\mathrm { P } _ { \mathrm { X } } = \mathrm { P } _ { \mathrm { Y } }$

A) $\mathrm { U } = \min ( \mathrm { X } , \mathrm { Y } )$
B) $U=X + X$
C) $\mathrm { U } = \mathrm { X } ^ { 1 / 2 } \mathrm { Y } ^ { 1 / 2 }$
D) $\mathrm { U } = \mathrm { X } ^ { 1 / 4 } \mathrm { Y } ^ { 3 / 4 }$