The line that uses the poetic element of alliteration to create an unfavorable or sinister effect is:
C. “What this grim, ungainly, ghastly, gaunt, and ominous bird of yore”
The repetition of the "g" and "o" sounds in "grim," "ungainly," "ghastly," "gaunt," and "ominous" contributes to a dark and eerie tone, enhancing the sinister atmosphere.