Final answer:
The A. gravitational force is what pulls objects with mass together. However, within an atom, it's the electric force that keeps the atom together, with the strong nuclear force also playing a role in the nucleus.
Step-by-step explanation:
The force that pulls objects together is called gravitational force (Option A). This force is an attraction that exists between all objects with mass. It is a fundamental force that operates at all scales, dominating at the largest distances and mass scales, such as the force keeping planets in orbit around stars or the effect on an apple falling from a tree.
When it comes to atoms, the force responsible for holding them together is not the gravitational force but the electric force (Option B). This is because atoms consist of electrically charged particles: protons, neutrons, and electrons. The electric force attracts opposite charges and repels like charges, thus preventing electrons from simply flying away from the nucleus. It's also worth noting that another vital force within the nucleus is the strong nuclear force, which helps bind protons and neutrons together, overcoming their electric repulsion.