To calculate the inflation rate from 2018 to 2019, you can use the following formula:
\[ \text{Inflation Rate} = \frac{\text{CPI in 2019} - \text{CPI in 2018}}{\text{CPI in 2018}} \times 100 \]
Substituting the given values:
\[ \text{Inflation Rate} = \frac{257 - 251}{251} \times 100 \]
\[ \text{Inflation Rate} = \frac{6}{251} \times 100 \approx 2.39% \]
Rounding to the nearest tenth of a percent, the inflation rate is approximately 2.4%.
Thus, the correct response is: 2.4%.