To find the cost price of a ball, follow these steps:

1. Define Variables:
- Let the cost price of one ball be $C$.
- The total cost price for 17 balls is $17C$.
- The total selling price for 17 balls is $720$.

2. Determine the Loss:
- The loss is given as the cost price of 5 balls, so:
$$\text{Loss}=5C$$

3. Use the Loss Formula:
- Loss is the difference between the total cost price and the selling price:
$$\text{Loss}=17C-720$$
- According to the given information, this loss equals the cost price of 5 balls:
$$17C-720=5C$$

4. Solve for $C$:
- Rearrange the equation to solve for $C$:
$$17C-5C=720$$
$$12C=720$$
$$C=\frac{720}{12}=60$$


The cost price of one ball is N60.

The correct answer is C. N60.