The reward systems that is likely to require sophisticated computer systems is:

A) skill-based pay
B) informal rewards
C) banking time off
D) flexible benefit plans

Flexible benefit plans require sophisticated computer systems to manage employee choices and deductions accurately. This system needs to track different benefit options, employee choices, and monetary values associated with each benefit, which can be efficiently managed through computer systems. Skill-based pay, informal rewards, and banking time off do not necessarily require sophisticated computer systems to operate.