Answer:
A. Alliteration.
Step-by-step explanation:
Alliteration is a literary figure of speech that occurs when the same sounding words or letters are adjacent to one another or in close quarters. This repetition of letters or identical initial consonant sounds are also known as rhymes/ rhyming words. They provide a smooth flow and easy linking of the words in the sentence.
The lines "Lay was lost" and "vein I boast [......] were vanquished by the mighty theme" from Sonnet XVIII uses this figurative speech where the consonant 'l' happens twice (lay and lost) in almost consecutive steps. Likewise, the 'v' in the second phrase/line in 'vein' and 'vanquished' is also an example of alliteration in the poem/ sonnet written by Petrarch.