Explanation:

- The bus's speed increases linearly: it starts at an initial speed and increases by 9 km/hr every hour.

- Total travel time is 11 hours, and the total distance covered is 572 km.

- To find the initial speed, calculate the total distance covered using the arithmetic sequence formula.

- For each hour \( n \), speed = initial speed + 9(n-1).

- Total distance = sum of speeds over each hour.

- The sum of an arithmetic sequence: \( S = \frac{n}{2} \times (2a + (n-1)d) \).

- Here, \( S = 572 \), \( n = 11 \), \( a = \text{initial speed} \), and \( d = 9 \).

$$ S = \frac{11}{2} \times (2a + (11-1) \times 9) = 572 $$

- Solve for \( a \):

$$ 11a + 495 = 572 $$

$$ 11a = 77 $$

$$ a = \boxed{7} $$

- Option:2, 7 is correct.