In the context of Quantum Field Theory (QFT), "Path Integral Formulation" is:

a) A method where the probability amplitudes for a particle's path are summed over all possible paths, weighted by the exponential of the action, providing an alternative to operator-based approaches.
b) A technique for solving differential equations of motion for classical fields.
c) A method to calculate particle decay rates and cross-sections.
d) A representation of particle interactions in terms of Feynman diagrams.