Final answer:
The phrase 'If I break my heart in my body, it shall up' best illustrates Theseus's determination, as it shows his resolve to lift the stone regardless of the risk to his own well-being.
Step-by-step explanation:
The detail that best reveals Theseus's determination in the passage is 'If I break my heart in my body, it shall up.' This statement shows that Theseus is willing to exert himself to his utmost limits, even if it means risking his life, to accomplish his goal of lifting the stone. It portrays a strong commitment to completing the task at hand no matter the personal cost, which is emblematic of determination. Despite the formidable challenge, he remains resolute and eventually succeeds, as indicated by the line 'he tugged at it once more, and lifted it.' is a literal demonstration of his determination coming to fruition.