On p. 17, the instant Law names himself “Trafalgar D. Water Law”, Corazon — who has feigned muteness for the entire flashback — breaks his silence specifically because of the “D.”
The payoff
Earliest known caller
(Not recorded.)
Why this matters
A classic Oda move: a long-silent character’s first words are triggered by a single loaded word, simultaneously paying off the “Corazon never speaks” presentation and planting a new, larger mystery (the danger of the “D.” to those near the Donquixote family). The “D.” that recurs around Law for the rest of the series gets its first in-text dread here. See Will of D, D.