Final answer:
The only setting that will cause significant deflection of alpha particles is an increase in the spread of positive charge.
Step-by-step explanation:
The only setting of the atom's "Spread of Positive Charge" that will cause alpha particles to be deflected significantly is a) Increase in the spread of positive charge.
When the charges are distributed over some area, there will be a concentration of charge along the side closest to the oppositely charged object. This concentration of charge increases the net force and leads to significant deflection of the alpha particles.