C for me
Peculiarities are a function of personal idiosyncrasies or pervelant usage during a time period.
If the test does not "...provide conclusive proof of the authorship of any poem?" it could then be because the pecularity is the sign of the widely shared norm/practice.
Certain minor peculiarities of language are used unconsciously by poets. If such peculiarities appear in the works of more than one poet, they are likely to reflect the language in common use during the poets’ time. However, if they appear in the work of only one poet, they are likely to be personal idiosyncrasies. As such, they can provide a kind of “fingerprint” that allows scholars, by comparing a poem of previously unknown authorship to the work of a particular known poet, to identify the poem as the work of that poet.
For which one of the following reasons can the test described above never provide conclusive proof of the authorship of any poem?
(A) The labor of analyzing peculiarities of language both in the work of a known poet and in a poem of unknown authorship would not be undertaken unless other evidence already suggested that the poem of unknown authorship was written by the known poet.
(B) A peculiarity of language that might be used as an identifying mark is likely to be widely scattered in the work of a poet, so that a single poem not known to have been written by that poet might not include that peculiarity.
(C) A peculiarity of language in a poem of unknown authorship could be evidence either that the poem was written by the one author known to use that peculiarity or that the peculiarity was not unique to that author.
(D) Minor peculiarities of language contribute far less to the literary effect of any poem than such factors as poetic form, subject matter, and deliberately chosen wording
(E) A poet’s use of some peculiarities of language might have been unconscious in some poems and conscious in other poems, and the two uses would be indistinguishable to scholars at a later date.