Identifying a Thousandth: Decimal Fraction Notation Exercise

To solve this problem, we will follow these steps:

• Step 1: Recognize that a "thousandth" refers to the fraction $\frac{1}{1000}$.
• Step 2: Convert the fraction $\frac{1}{1000}$ to its decimal representation.
• Step 3: Determine which of the given options matches this decimal representation.

Now, let's examine each step:
Step 1: A fraction representing a thousandth is $\frac{1}{1000}$.
Step 2: When we convert $\frac{1}{1000}$ into a decimal, we see that it equals $0.001$ because it requires moving the decimal three places to the left, indicating three zeros before the digit 1.
Step 3: Among the choices provided, option 4 identifies a decimal representation of $0.001$, which correctly reflects a single thousandth place.

Therefore, the solution to the problem is $0.001$.