The best answer choice is C: "While failing to describe his lover's beauty, he still manages to flatter her." Petrarch's "Sonnet 18" is about Laura's beauty and the way it evades proper description. As such, Petrarch feels he is not equipped to truly praise Laura in the way she deserves to be praised.
In the first couple of lines, Petrarch admits,
Ashamed sometimes thy beauties should remain
As yet unsung, sweet lady, in my rhyme;
Here, Petrarch expresses the idea that it is a shame Laura's beauty should not be immortalized in poetry. He can remember the first time he saw her, as he recounts in lines 3-4, yet he does not feel he can adequately capture her beauty in words. He continues,
But no fit polish can my verse attain,
Not mine is strength to try the task sublime: (5-6)
Petrarch laments that he cannot produce a poem as "sublime" as Laura's beauty. He just feels as though he cannot get it quite right. He does not have the ability or the "strength" to write a poem that can do Laura justice.
(The entire section contains 4 answers and 781 words.)