To find out how many miles Sonny walks after seven days, you can multiply the distance he walks each day by the number of days:
\[ \text{Distance walked in 7 days} = \left(\frac{2}{3} \text{ miles/day}\right) \times 7 \text{ days} \]
Calculating this:
\[ = \frac{2}{3} \times 7 = \frac{2 \times 7}{3} = \frac{14}{3} \text{ miles} \]
So, after seven days, Sonny walks \( \frac{14}{3} \) miles, which is equivalent to \( 4 \frac{2}{3} \) miles.
From the provided options, "14 thirds miles" (which is \( \frac{14}{3} \) miles) is correct.