The "Standard Model" of particle physics, which unifies the electromagnetic, weak, and strong nuclear forces, relies on the concept of:

a) Gauge invariance, where the fundamental interactions are mediated by gauge bosons associated with symmetry groups. The model describes particles such as quarks and leptons, and their interactions through exchange particles like photons, W/Z bosons, and gluons.
b) The equivalence principle to describe gravitational interactions.
c) The principle of least action in classical mechanics.
d) The wave-particle duality of electrons in atomic systems.