Correct choice is (a) Linear triangular element

To explain I would say: Mesh convergence determines how many elements are required in a finite element model to ensure that the results of an analysis are not affected by varying the size of the mesh. Once a mesh is converged, no change is observed in the results even after changing its density. The linear triangular element mesh has the slowest convergence compared to the quadratic triangular element, linear and quadratic rectangular elements.