Explanation:

Let's evaluate the options to determine which signs should be interchanged.

- Option 1: + and ×

- New equation: 15 × 5 - 10 + 8 ÷ 4.

- Solve as: 75 - 10 + 2 = 67 (Not 15).

- This does not make the equation correct.

- Option 2: × and -

- New equation: 15 + 5 × 10 - 8 ÷ 4.

- Solve as: 15 + 50 - 2 = 63 (Not 15).

- This does not make the equation correct.

- Option 3: ÷ and +

- New equation: 15 ÷ 5 - 10 × 8 + 4.

- Solve as: 3 - 80 + 4 = -73 (Not 15).

- This does not make the equation correct.

- Option 4: ÷ and -

- New equation: 15 + 5 - 10 × 8 - 4.

- Solve as: 15 + 5 ÷ 4 × 8 - 10 = 15.

- This makes the equation correct.

The signs in Option 4 are correct to interchange.