Final answer:
The line from "Those Winter Sundays" that demonstrates the poet’s understanding of his father's love is "No one ever thanked him," which acknowledges the silent and unappreciated acts of love.
Step-by-step explanation:
The line from Robert Hayden's Those Winter Sundays that demonstrates the poet's realization and understanding of his father's love is option b) "No one ever thanked him." This line highlights the father's unappreciated efforts and the humility with which he carried out his duties, reflecting a deeper understanding of love that is often unrecognized and unspoken. The poet's retrospective recognition of his father's love and sacrifice is conveyed through the personal reflection and evocative language used in the poem.