The correct option is (a) Iso-parametric

For explanation: In sub-parametric formulations, the geometry is represented by elements of a lower order than those used to approximate the dependent variable. An example of this category is provided by the beam element. In iso-parametric formulations (which are the most common in practice), the same element is used to approximate the geometry as well as the dependent unknowns. In super-parametric formulations, the geometry is represented by elements of a higher order than those used to approximate the dependent variables.