Answer:
C) the project's payback period will be unchanged.
Step-by-step explanation:
Payback period is the time period in which initial investment of the asset recovered from it benefit. In this Question the Initial Investment is recovered in year 4, the cash flows of year 5 and year 6 will not effect the payback period. It remains the same because the cash flows of first 4 years remains same. The change in the first 4 years would impact the payback period. With constant discount rate the payback period will remain unchanged.