Final answer:
Option (a), In the context of the provided lines, 'them' likely refers to 'lines,' as there is a thematic focus on the continuity and flow of rivers which can be poetically represented as lines.
Step-by-step explanation:
In the provided lines, the pronoun 'them' most likely refers to 'lines' when considering the literary context and the specific usage within the sentences. This can be concluded from the references made to the 'line' in the poetry and prose provided, which frequently associate the river and its elements with the theme of continuity and flow.
For example, the phrase 'The eastern member of the two great rivers that define Mesopotamia', suggests that rivers, and thus the lines they form, carry a significant meaning.