To solve this question, we can use the concept of man-days of work.
Let's calculate the total man-days required to build the wall:
17 men working for 24 days = 17 * 24 = 408 man-days.
Now, let's calculate the work done by the 17 men in one day:
Work done in one day = Total man-days / Total days = 408 / 24 = 17 man-days.
Next, let's calculate the work done by one man in one day:
Work done by one man in one day = Work done in one day / Number of men = 17 / 17 = 1 man-day.
We are given that the hours per day are reduced in the ratio 5:4. This means that the work done by one man in one day is also reduced in the same ratio.
Let's calculate the work done by one man in one day after the reduction:
Reduced work done by one man in one day = Work done by one man in one day * (4/5) = 1 * (4/5) = 4/5 man-days.
Now, let's calculate the number of days required for 18 men to complete the work:
Total man-days required = Number of men * Number of days.
Number of days = Total man-days required / Number of men = 408 / 18 = 22.67 days.
However, we need to account for the reduction in hours per day.
Let's calculate the actual number of days required after the reduction:
Actual number of days required = Number of days * (5/4) = 22.67 * (5/4) = 28.33 days.
Therefore, the correct answer is B) 28.33 days.