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The problem asks to determine the approximate amount of sodium iodide-131 remaining in the body after 18 hours, given that sodium iodide-131 has a half-life of 1.8 hours.

### Step-by-step approach:

1. **Calculate the number of half-lives that occur in 18 hours:**
   \[
   \text{Number of half-lives} = \frac{\text{Total time (in hours)}}{\text{Half-life duration (in hours)}} = \frac{18}{1.8} = 10
   \]

2. From the table, it shows the amount of sodium iodide-131 after 5 half-lives. To calculate the amount after 10 half-lives, we notice that for each half-life, the amount of the substance remaining is halved.

   After 5 half-lives, the amount is **8.688** (from the table). To find the amount after another 5 half-lives (total 10 half-lives), we can continue halving the value:
   
   - After 6th half-life: \( 8.688 \div 2 = 4.344 \)
   - After 7th half-life: \( 4.344 \div 2 = 2.172 \)
   - After 8th half-life: \( 2.172 \div 2 = 1.086 \)
   - After 9th half-life: \( 1.086 \div 2 = 0.543 \)
   - After 10th half-life: \( 0.543 \div 2 = 0.271 \)

Thus, after 10 half-lives, the approximate amount of sodium iodide-131 remaining in the body is **0.271**.

### Final Answer:
- The correct option is **D. 0.271**.

Would you like more details or have any further questions?

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Here are some related questions to consider:

1. How would the result change if the half-life of sodium iodide-131 were longer, say 3 hours?
2. What is the decay rate formula for a radioactive substance, and how does it apply here?
3. How much sodium iodide-131 would remain after only 9 hours?
4. How do you calculate the percentage of the original amount remaining after 10 half-lives?
5. What practical applications are there for understanding the half-life of a substance in medical treatments?

**Tip:** The exponential decay formula is helpful when working with half-lives: \( A(t) = A_0 \times \left(\frac{1}{2}\right)^{\frac{t}{t_{1/2}}} \), where \( A_0 \) is the initial amount, \( t \) is time, and \( t_{1/2} \) is the half-life.

What approximate amount of sodium iodide-131 will remain in the body after 18 hours?

Learn how to calculate the remaining amount of sodium iodide-131 in the body after 18 hours using the half-life concept. This problem covers key topics such as exponential decay and radioactive half-life calculations, suitable for high school students in Grades 10-12.

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