To find the length of the small youth T-shirt based on the width given, we need to consider the size ratio between the large and small sizes.
The large T-shirt is 21.5 inches wide. The small youth T-shirt is 15.5 inches wide.
First, we determine the ratio of the width of the small T-shirt to the width of the large T-shirt:
\[ \text{Ratio} = \frac{\text{Width of Small T-shirt}}{\text{Width of Large T-shirt}} = \frac{15.5}{21.5} \]
Now we calculate this ratio:
\[ \frac{15.5}{21.5} \approx 0.7209 \]
Next, we apply this ratio to the length of the large T-shirt, which is 26.5 inches:
\[ \text{Length of Small T-shirt} = 0.7209 \times 26.5 \approx 19.1 \text{ inches} \]
Therefore, the length of the small youth T-shirt is approximately 19.1 inches.
So, the best answer from the options provided is 19.1 inches.