In the early part of the Modern English period the vocabulary was enlarged by
the widespread use of one part of speech for … and by increased borrowings from other languages.
(a) the others
(b) the other
In the beginning, I have thought the correct version is (a) since there are 8 (9 in some sources) parts of speech besides one part of speech (boldfaced words to which I am referring to). However, my teacher told me that the correct answer would be (d). But he could not ground his version.
Please could you explain why (d) is only correct option here?