To determine the score of the fourth boy, let`s analyze the given information.

First, we are told that the average score of three boys in Mathematics is 30. This means that the total scores of the three boys combined is 90 (30 * 3).

Next, when the score of the fourth boy is added to the total score of the three boys, the average is lowered to 25. This means that the total scores of all four boys combined is 100 (25 * 4).

Now, to find the score of the fourth boy, we can subtract the total score of the three boys (90) from the total score of all four boys (100). The difference will give us the score of the fourth boy, which is 10 (100 - 90).

Therefore, the score of the fourth boy is 10.

Note: All other options are incorrect as they do not satisfy the given conditions.