I've just been revising waves and I don't fully understand this question. I understand that as it is a transverse wave the particles are oscillating perpendicular to the direction of motion of the wave however I'm not exactly sure about the motion of B. Is it just going up and down about the equilibrium position (up to max +ve displacement, back through equilibirum, down to max -ve displacement then back up to equilibrium) while the wave moves to the right?