Final answer:
The line from Shakespeare's 'Sonnet 18' contemplates immortalizing the subject's beauty through poetry, transcending the temporal limits of physical existence.
Step-by-step explanation:
The line "When in eternal lines to time thou growest:" from Shakespeare's Sonnet 18 captures the essence of the poem's theme of immortality through verse. The analysis of this line falls into the category of signifying the immortality of the subject in the poet's verse (option a). In this context, 'eternal lines' refers to the enduring quality of the written word, particularly the sonnet itself, which Shakespeare believes will grant eternal life to the beauty of the subject he addresses. The poem expresses that despite the inevitable passing of time and the physical decay associated with aging, the subject's beauty and essence will live on through the poetry - thus defying the constraints of time and mortality.