Step 1: Understanding the Concept:
This question requires identifying the source poem of a given line of text.
Step 2: Detailed Explanation:
The line "Thou watchest the last oozings hours by hours" is from the third and final stanza of John Keats' famous poem, "Ode to Autumn". In this stanza, the poet is describing the final stages of autumn, and the line specifically refers to watching the slow process of a cider-press squeezing the last drops from apples.
Step 3: Final Answer:
The given line is a quote from "Ode to Autumn" by John Keats. Therefore, option (C) is the correct answer.