The correct answer is D. 1000.
The series is a list of cubes of the first 10 natural numbers. The first number in the series is 1, which is the cube of 1. The second number in the series is 8, which is the cube of 2. The third number in the series is 27, which is the cube of 3. The fourth number in the series is 64, which is the cube of 4. The fifth number in the series is 125, which is the cube of 5. The sixth number in the series is 216, which is the cube of 6. The seventh number in the series is 343, which is the cube of 7. The eighth number in the series is 512, which is the cube of 8. The ninth number in the series is 729, which is the cube of 9. The tenth number in the series is 1024, which is the cube of 10.
1000 is not a cube of any natural number. Therefore, it is not a number in the series.