Final answer:
Sonnet 79 explores themes around love and poetic expression, with the poet's verses losing grace as the muse shifts attention, and the new poet inventing beauty while basing praises on the subject's behavior and cheek. These actions evoke a bitter tone, as the original poet's work is overshadowed by the new poet's flattery.
Step-by-step explanation:
The main theme of Sonnet 79 is not clearly stated among the options, but based on Renaissance love poetry, it is often a case of unrequited love or explorations of love itself. As the muse gives place to another, the poet's verses lose grace and decay.
The poet believes that sweet love deserves a worthier pen for their lovely argument. The new poet invents beauty about the subject of the sonnet.
This beauty and virtue are found in the subject's behavior and cheek, not from the poet's previous work or outside influences like nature or ancient texts.
However, the praises given by the new poet are not fully endorsed by the speaker, suggesting that the subject deserves even more.
The overall tone of Sonnet 79 could be determined as bitter, due to the apparent depreciation of the original poet's work in light of the new poet's flattery. In modern interpretation, the phrase "He robs thee of and pays it thee again" implies that the new poet steals from the subject but compensates with praise.