The correct term for when unlike charges pull together in a straight line is 'Attraction'. This is based on Coulomb's law, which indicates that opposite charges exert an attractive force towards each other.
The term that best goes with the definition 'is when unlike charges pull together in a straight line' is (b) Attraction. This is because Coulomb's law states that opposite, or unlike, charges attract each other. When a negative and a positive charge come into proximity, they will exert an attractive force towards each other, causing them to pull together. This is a foundational concept in the study of electromagnetic forces, where it is understood that like charges repel and unlike charges attract.