Explanation:

Let's take a look at the situation and calculations step by step:

- Initial Volume of the Cube: The cube has a side of 15 cm. So, its volume is $15\times 15\times 15=3375{\text{ cm}}^3$.

- Dimensions of the Container: The container has a length of 25 cm, breadth of 20 cm, and a height of 18 cm.

- Rise in Water Level Calculation: When the cube is submerged, it displaces an amount of water equal to its volume. To find the rise in water level ($h$), use the formula for volume displacement:

$$

\text{Volume of cube} = \text{Length} \times \text{Breadth} \times h

$$

$$

3375 = 25 \times 20 \times h

$$

$$

h = \frac{3375}{500} = 6.75 \text{ cm}

$$

- Conclusion: The water level rises by 6.75 cm.

- Correct Option: The correct answer is Option 4: 6.75 cm