The professor's theory of there being several families of distinct Native American languages found in the American Northwest has become the basis of modern linguistic research in the field.
A. of there being several families of distinct Native American languages found in the American Northwest has become the basis of modern linguistic research in the field
"of there being" wrong usage of being
B. of several families of distinct Native American languages that are found in the American Northwest has become the basis of modern linguistic research in the field
"Professor's theory of several families" This option changes the meaning
C. that several families of distinct Native American languages are found in the American Northwest has become the basis of modern linguistic research in the field Correct
D. which is that several families of distinct Native American languages are found in the American Northwest has become the basis of modern linguistic research in the field
Usage of "which is" redundant
E. that has become the basis of modern linguistic research in the field is that several families of distinct Native American languages are found in the American Northwest
Changing the position of the modifier changes the meaning of the sentence. In the original sentence "that several families of distinct Native American languages are found in the American Northwest" is the modifier for "the theory", however in this option "that has become the basis of modern linguistic research in the field" becomes the modifier.