Let the unbroken part be x and the broken part be y; using trigonometric ratios, tan(30 degrees) equals x divided by 15, making x equal to 15 divided by the square root of 3. Also, cos(30 degrees) equals 15 divided by y, so y equals 15 multiplied by the square root of 3 divided by 3, which is 5 multiplied by the square root of 3. The present height of the broken tree is x plus y, which is 15 divided by the square root of 3 plus 5 multiplied by the square root of 3, yielding 10 multiplied by the square root of 3 m. As this exact value is not listed among the options, the correct choice is None of these.