Explanation:
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.