Benjamin bought a motorcycle for GH₵16,800.00 and later sold it at 85% of the original cost. He spent GH₵3,930.75 from the money he received from the sale of the motorcycle and invested the remaining at 18% per annum simple interest. How much did he earn in 9 years?

The motorcycle was bought for GH¢16,800.00 and sold at 85% of the original cost.

$$\text{Selling Price} = 0.85 \times \text{GH¢}16,800.00$$ $$\text{Selling Price} = \text{GH¢}14,280.00$$

Benjamin received GH¢14,280.00 from the sale and spent GH¢3,930.75. The remaining amount was invested.

$$\text{Amount Invested (Principal, P)} = \text{GH¢}14,280.00 - \text{GH¢}3,930.75$$ $$\text{P} = \text{GH¢}10,349.25$$

The principal (P) is GH¢10,349.25, the interest rate (R) is 18% per annum (0.18), and the time (T) is 9 years.
The formula for simple interest is $I = P \times R \times T$.

$$I = \text{GH¢}10,349.25 \times 0.18 \times 9$$ $$I = \text{GH¢}10,349.25 \times 1.62$$ $$I = \text{GH¢}16,765.785$$

Rounding GH¢16,765.785 to two decimal places gives GH¢16,765.79.
This matches option C.

The amount Benjamin earned in 9 years is the simple interest.
The correct option is C.

$$\boxed{\text{GH¢}16,765.79}$$