The correct answer is indeed 4, "Widgeon," and here's why:
Seamus Heaney’s poem "Widgeon" is dedicated to the Irish poet Paul Muldoon. This dedication reflects the admiration Heaney had for Muldoon, who is an influential contemporary poet and has been noted for his innovative and distinctive style.
In contrast, the other poems listed—“The Loaning,” “The Sandpit,” and “A Migration”—are not known to be dedicated to Paul Muldoon. Therefore, "Widgeon" is the specific poem in which Heaney acknowledges Muldoon's impact, making option 4 the correct choice.