Final answer:
In Act 1 of The Crucible, John Proctor's significant action is remaining silent during the accusations, which highlights his character's internal conflict and discomfort with the Salem witch trials.
Step-by-step explanation:
A significant action taken by John Proctor in Act 1 of The Crucible is d) Remaining silent during the accusations. During this act, Proctor does not accuse anyone of witchcraft or protest against the trials; nor does he confess to witchcraft. His silence reflects his internal conflict and sets up his character development for the rest of the play. Proctor's silence can be seen as a sign of his skepticism towards the frenzy of accusations and his discomfort with the unfolding situation in Salem, partially due to his own secrets.