What causes a proton and neutron to come together?

A strong nuclear force theres no other way to explain it

Protons and neutrons are held together in a nucleus of an atom by the strong force. The strong force gets it name by being the strongest attractive force. It is 137 times more powerful than electromagnetic, which by the way cannot hold neutrons to protons because neutrons are not charged. It is 100,000 times more powerful than the weak force and 6,000 billion billion billion billion (6 followed by 39 zeroes) times more powerful than gravity which by the way has almost no effect at atomic scales.According to the standard model of particle physics, the fundamental forces (strong, weak, electromagnetic and gravity) are predicted to occur as a result of an exchange between particles via "force carrying particles". Also, neutrons and protons are made up of tinier particles called quarks. And it is the quarks that exchange force carrying particles between each other to give rise to the strong force. The force carrying particles are called gluons.It should be mentioned that the strong force only operates at EXTREMELY small distances. These distances are on the order of a 1000th millionth millionth of a meter (10 to the power of -15). If you think about a micrometer (one hundredth the size of a human hair), it is a billion times smaller than that.The strong force also attracts protons to protons or neutrons to neutrons. In the case of protons to protons, the strong force loses strength after the distance mentioned above and succumbs to the electromagnetic force which pushes the protons apart. In this case the force carrier of electromagnetics is the photon (constituent of light).

Dmight be the correct answer