Final answer:
The item that best describes the bomb effect is The Common Cold, which grants Isaac poison tears that deal damage over time.
Step-by-step explanation:
The item that best describes the bomb effect that inflicts poison damage on enemies caught within the blast radius, adding +5 bombs and a Damage over Time (DoT) effect that deals either 4 or 6 damage per tick, is The Common Cold.
The Common Cold is an item in the game The Binding of Isaac that grants Isaac spectral tears and a chance to fire poison tears that deal damage over time. This means that enemies hit by the poison tears will continue to take damage after they are hit.
The addition of +5 bombs and the poison damage makes The Common Cold the best choice among the given options.