Here's a problem to solve: The following mathematical expressions represent four different concentrations of a chemical solution to be used in a science experiment. Which one is equal in magnitude to 1/1000?

Among the 4 following options, which is the correct answer?

- A: 1.0 x 10^3
 
- B: 1.0 x 10^4
 
- C: 1.0 x 10^-3
 
- D: 1.0 x 10^-4
 