In a triangle,

A)a+b=c B)a-b=c C)b+c=a D)b=c

we cant say,because you dont specify a particular triangle.but if your options are taken as cases then D is the right case.

sum of two sides is always greater than third side....so option a and c are not possible..and also the third side is greater than the difference of two sides...so b is also not possible...thus only one option is left i.e. d and the case will be of isosceles triangle...