To find the number of times in a day when the hands of a clock are in a straight line but opposite in direction, we need to understand the movement of the clock hands.

The hour hand of a clock completes one full revolution in 12 hours, while the minute hand completes one full revolution in 60 minutes. Therefore, the relative speed of the minute hand with respect to the hour hand is 12 times faster.

Let's analyze the different positions of the clock hands:

1. Straight Line at 12 o'clock:
- The hands are in a straight line but not opposite in direction.

2. Straight Line at 1 o'clock:
- The minute hand moves slightly ahead of the hour hand.
- The hands are not in a straight line but opposite in direction.

3. Straight Line at 2 o'clock:
- The minute hand moves further ahead of the hour hand.
- The hands are not in a straight line but opposite in direction.

4. Straight Line at 3 o'clock:
- The minute hand is at 12, while the hour hand is at 3.
- The hands are in a straight line but not opposite in direction.

5. Straight Line at 4 o'clock:
- The hour hand starts moving towards the 4 while the minute hand is at 12.
- The hands are not in a straight line but opposite in direction.

6. Straight Line at 5 o'clock:
- The hour hand moves closer to the 5 while the minute hand is at 12.
- The hands are not in a straight line but opposite in direction.

7. Straight Line at 6 o'clock:
- The hour hand is at 6, while the minute hand is at 12.
- The hands are in a straight line but not opposite in direction.

8. Straight Line at 7 o'clock:
- The hour hand moves slightly ahead of the 7 while the minute hand is at 12.
- The hands are not in a straight line but opposite in direction.

9. Straight Line at 8 o'clock:
- The hour hand moves further ahead of the 8 while the minute hand is at 12.
- The hands are not in a straight line but opposite in direction.

10. Straight Line at 9 o'clock:
- The minute hand is at 12, while the hour hand is at 9.
- The hands are in a straight line but not opposite in direction.

11. Straight Line at 10 o'clock:
- The minute hand starts moving towards the 10 while the hour hand is at 9.
- The hands are not in a straight line but opposite in direction.

12. Straight Line at 11 o'clock:
- The minute hand moves closer to the 11 while the hour hand is at 9.
- The hands are not in a straight line but opposite in direction.

13. Straight Line at 12 o'clock (next day):
- The hands are in a straight line but not opposite in direction.

From the analysis above, we can see that the hands of a clock are in a straight line but opposite in direction only 11 times in a 12-hour cycle. Since there are 24 hours in a day, the hands will be in a straight line but opposite in direction 22 times in a day.

Therefore, the correct answer is option B