Final answer:
Interest rates is the correct option to transform unequal lives into equal lives.
Step-by-step explanation:
b. Interest rates
The process of transforming cash flows from different periods to make them comparable is known as discounting or compounding, and it involves the application of interest rates. By discounting future cash flows back to their present value or compounding present values to a future equivalent, analysts can adjust for the time value of money, allowing for meaningful comparisons between alternatives with different time horizons. Options like depreciation (option a) are accounting methods for allocating the cost of an asset over its useful life, not for adjusting cash flows over time. Net present value (option c) is a financial metric that already considers the time value of money by discounting future cash flows. Taxation (option d) is related to the treatment of income for tax purposes and doesn't directly transform cash flows to equal lives.