Explanation:

- The original total score for the class of 45 students is 45 * 52 = 2340.

- A group of 6 boys with an average score of 40 leaves. Their total score is 6 * 40 = 240.

- When they leave, the new total score becomes 2340 - 240 = 2100 for 39 students.

- Another group of 8 boys joins with an average score of 43. Their total score is 8 * 43 = 344.

- Now, adding their scores, the total score is 2100 + 344 = 2444 for 47 students.

- Therefore, the new average score is 2444 / 47 ˜ 52.

- All calculations show that Option 4 (52) is the correct answer.