Step 1 - Understanding fundamental forces
There are four fundamental forces in the universe: eletromagnetic force, strong nuclear force, weak force and gravity.
Eletromagnetic force mediates the interaction between charged particles and magnetic fields, whereas gravity mediates the interaction between mass.
Weak and strong nuclear forces operate more at the subatomic level. The strong nuclear force is responsible for keeping protons and neutros together in the nucleus.
Since all protons are positively charged, there's an intense electromagnetic repulsion between them, which must me counterbalanced by the strong nuclear force.
Step 2 - Answering the exercise
What does the strong nuclear force do?
Answer: It keeps protons and neutrons together in the nucleus