The Triangle Inequality Theorem lets us know that the sum of the two shortest sides of the triangle must be greater than the third side of the triangle.
In both A and D, the sum of the shortest two sides are equal to, not greater than the third side, so they will not form a triangle.
In B, 12+20 is 32, which is greater than 25. And in C, 18+24 is 42, which is greater than 30, so they both will form a triangle.
