Final answer:
Wigner's friend experiment focuses on wavefunction collapse in quantum mechanics, presenting a paradox when quantum rules are applied to observers.
Step-by-step explanation:
The main focus of Wigner's friend thought experiment, introduced by Eugene Wigner in 1961, is d) Wavefunction collapse in quantum mechanics.
The experiment questions the objectivity of a quantum measurement by considering a scenario where an observer, Wigner's friend, performs a measurement inside a laboratory, and Wigner observes from outside. At the core, it illustrates the paradox that arises when applying the rules of quantum mechanics to observers themselves and highlights issues of consciousness within the framework of quantum theory.