Final answer:
In The Binding of Isaac, the Iron Bar item increases damage and provides a chance to concuss enemies, with the chance affected by the player's luck stat.
Step-by-step explanation:
The item in The Binding of Isaac that provides the effect of "+0.3 Damage Up" and a chance to concuss enemies is A) Iron Bar. When an enemy is hit by a tear from Isaac after he has this item, they can become dazed and walk around confused for a short period. The likelihood of this concussion effect is influenced by the player's luck stat, and with +27 Luck, the effect will occur with each tear that hits an enemy.