GRE Quantitative Comparison asks you to compare. In general, these questions do not require difficult calculation. Do not assume anything that isn't stated in the problem.
The special triangles in this case are 3 4 5 and 12 13 15. That would make the second triangle's third side larger. But it doesn't apply in this case. The problem didn't state that the triangles are right triangles. Without that information, you cannot assume they are special triangles.
The only information the problem provides is that these are triangles. In any triangle the sum of two sides must be larger than the third side. That's the triangle inequality theorem.
For the first one with 3 and 4, the third side s must be greater than 1. If that third side is less than or equal to 1, then 3+s is not > 4 and it's not a triangle.
For the second one with 12 and 13, the third side s must be greater than 1. If that third side is less than or equal to 1, then 12+s is not > 13.
Because that's the only thing known, that in each case the third side must be > 1, the GRE answer is D - it cannot be determined without further information.
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