The correct answer is: B. London theory.
The London theory is a phenomenological theory of superconductivity that was developed by brothers Fritz and Heinz London in 1935. It is based on the assumption that superconductivity is caused by the formation of Cooper pairs, which are pairs of electrons that are bound together by the exchange of virtual bosons. The London theory predicts that superconductors have zero electrical resistance and expel magnetic fields, which are the two defining properties of superconductivity.
The Ginzburg-Landau theory is a more sophisticated theory of superconductivity that was developed by Lev Landau and Vitaly Ginzburg in 1950. It is based on the assumption that superconductivity is caused by a phase transition in which the superconducting state is a different phase of matter than the normal state. The Ginzburg-Landau theory predicts a number of properties of superconductors, including the critical temperature, the critical field, and the coherence length.
The resonating valence bond theory is a theory of superconductivity that was developed by Philip Anderson in 1987. It is based on the assumption that superconductivity is caused by the formation of resonating valence bonds, which are pairs of electrons that are bound together by the sharing of electrons. The resonating valence bond theory predicts that superconductors have a high critical temperature and are resistant to disorder.
Quantum field theory is a theory of physics that is based on the principles of quantum mechanics and field theory. It is used to describe the behavior of elementary particles and their interactions. Quantum field theory is not a theory of superconductivity.