Calculating return on investment for an investment center is defined by the following formula:

A) Contribution margin/Ending assets.
B) Gross profit/Ending assets.
C) Net income/Ending assets.
D) Income/Average invested assets.
E) Contribution margin/Average invested assets.

Return on investment (ROI) is calculated by dividing the income earned by the investment center by the average invested assets over the same period. "Income" in this context could refer to net income or operating income. Therefore, the formula for ROI is Income/Average invested assets. Choice D is the only option that includes the appropriate formula for calculating ROI.