After reviewing the code that implements the Itr abstract class, the following method:

public Itr iterator(){ return new DArrayItr(this); }

a: is invalid because the statement \"new DArrayItr(this)\" returns an object mismatched with return data type \"Itr\"

b: it's valid

c: is invalid because the \"this\" in the statement \"new DArrayItr(this)\" is not a valid argument for \"DArrayItr\" constructor

d: is invalid because the statement because the \"\" in return data type \"Itr\" is not defined