Explanation:

To explain why the correct answer is option 'C', let's break down the given options and evaluate them one by one.

Option 'A': -i
Option 'B': -i
Option 'C': -i
Option 'D': -i

All the given options have the same value of '-i', which represents the imaginary unit in mathematics. The imaginary unit 'i' is defined as the square root of -1.

In mathematics, the square root of a negative number is not a real number, as the square of any real number is always a non-negative value. Therefore, the square root of -1 is not a real number, but an imaginary number.

The imaginary unit 'i' is commonly used to represent complex numbers, which are numbers of the form a + bi, where 'a' and 'b' are real numbers, and 'i' is the imaginary unit.

In the given question, all the options represent the imaginary unit '-i'. The negative sign indicates that the imaginary unit is in the opposite direction of the positive imaginary unit 'i'.

Hence, the correct answer is option 'C', which represents '-i'.

In summary:
- The given options represent the imaginary unit '-i'.
- The imaginary unit 'i' is defined as the square root of -1.
- The negative sign indicates the direction of the imaginary unit in the opposite direction of the positive imaginary unit 'i'.
- Therefore, the correct answer is option 'C', which represents '-i'.