There are 2 answers: Choice B, Choice D
Why is this? Choice B is true because of the Pythagorean theorem. If vectors u and v are perpendicular, then we can form a rectangle. The diagonals of the rectangle are each of length ||u+v|| units long. From here you can form a right triangle with vector u+v as the hypotenuse. The expression shown in choice B is of the form c = sqrt(a^2+b^2), though with substitutions applied.
Choice D is true because of the triangle inequality theorem. This theorem says that the sum of any two sides of a triangle will always be larger than the third side. Otherwise, a triangle is not possible. I recommend cutting out strips of paper of various lengths and trying it out for yourself.
Choices A and C can be eliminated as they contradict B and D. Choice A contradicts choice D. Choice C contradicts choice B.