To calculate the time required for a 605-mile trip at an average speed of 55 miles per hour, you can use the formula:
\[ \text{Time} = \frac{\text{Distance}}{\text{Speed}} \]
Plugging in the values:
\[ \text{Time} = \frac{605 \text{ miles}}{55 \text{ miles per hour}} \approx 11 \text{ hours} \]
Thus, the time that should be allowed for the trip is approximately 11 hours.
The correct answer is D) 11 hours.