The weight of a U.S. penny is approximately 2.5 grams. To convert grams to pounds, you can use the conversion factor that 1 pound is approximately 454 grams.
Here's the calculation:
\[ \text{Weight in pounds} = \frac{2.5 \text{ grams}}{454 \text{ grams/pound}} \approx 0.00551 \text{ pounds} \]
This is approximately \( 5.51 \times 10^{-3} \) pounds.
Among the given options, the closest estimate to this value is:
B. \( 5 \times 10^{-3} \text{ lb} \)
So, the reasonable estimate for the weight of a U.S. penny is B.