In the FEM element library, what is the correct name for a three noded triangular element?

(a) Linear strain triangular element

(b) Constant strain triangular element

(c) Variable strain triangular element

(d) Higher-order triangular element

This question was posed to me by my school teacher while I was bunking the class.

The origin of the question is Library of Elements and Interpolation Functions in chapter Interpolation Functions, Numerical Integration and Modelling Considerations of Finite Element Method