Among all fractions (whose numerator and denominator are positive integers) strictly between $\tfrac{6}{17}$ and $\tfrac{9}{25}$, which one has the smallest denominator?
To find the fraction with the smallest denominator that lies strictly between $\frac{6}{17}$ and $\frac{9}{25}$, we can use the Farey sequence properties and mediant property of fractions.

1. **Convert the given fractions to decimal form for better understanding:**
   \[
   \frac{6}{17} \approx 0.352941 \quad \text{and} \quad \frac{9}{25} = 0.36
   \]

2. **Identify the mediant of the two fractions:**
   The mediant of two fractions $\frac{a}{b}$ and $\frac{c}{d}$ is given by:
   \[
   \frac{a+c}{b+d}
   \]
   Applying this to our fractions:
   \[
   \frac{6+9}{17+25} = \frac{15}{42}
   \]
   Simplify $\frac{15}{42}$:
   \[
   \frac{15}{42} = \frac{5}{14}
   \]

3. **Check if $\frac{5}{14}$ lies between $\frac{6}{17}$ and $\frac{9}{25}$:**
   Convert $\frac{5}{14}$ to decimal form:
   \[
   \frac{5}{14} \approx 0.357143
   \]
   Since $0.352941 < 0.357143 < 0.36$, $\frac{5}{14}$ indeed lies between $\frac{6}{17}$ and $\frac{9}{25}$.

4. **Verify that $\frac{5}{14}$ has the smallest denominator:**
   We need to ensure that there is no fraction with a smaller denominator that lies between $\frac{6}{17}$ and $\frac{9}{25}$. Fractions with denominators smaller than 14 are:
   \[
   \frac{1}{2}, \frac{1}{3}, \frac{2}{3}, \frac{1}{4}, \frac{3}{4}, \frac{1}{5}, \frac{2}{5}, \frac{3}{5}, \frac{4}{5}, \frac{1}{6}, \frac{5}{6}, \frac{1}{7}, \frac{2}{7}, \frac{3}{7}, \frac{4}{7}, \frac{5}{7}, \frac{6}{7}, \frac{1}{8}, \frac{3}{8}, \frac{5}{8}, \frac{7}{8}, \frac{1}{9}, \frac{2}{9}, \frac{4}{9}, \frac{5}{9}, \frac{7}{9}, \frac{8}{9}, \frac{1}{10}, \frac{3}{10}, \frac{7}{10}, \frac{9}{10}, \frac{1}{11}, \frac{2}{11}, \frac{3}{11}, \frac{4}{11}, \frac{5}{11}, \frac{6}{11}, \frac{7}{11}, \frac{8}{11}, \frac{9}{11}, \frac{10}{11}, \frac{1}{12}, \frac{5}{12}, \frac{7}{12}, \frac{11}{12}, \frac{1}{13}, \frac{2}{13}, \frac{3}{13}, \frac{4}{13}, \frac{5}{13}, \frac{6}{13}, \frac{7}{13}, \frac{8}{13}, \frac{9}{13}, \frac{10}{13}, \frac{11}{13}, \frac{12}{13}
   \]
   None of these fractions lie between $\frac{6}{17}$ and $\frac{9}{25}$.

Therefore, the fraction with the smallest denominator that lies strictly between $\frac{6}{17}$ and $\frac{9}{25}$ is $\boxed{\frac{5}{14}}$.