Answer: C. Adjacent antinodes separated by a displacement node oscillate in antiphase: when one is at maximum positive displacement, the other is at maximum negative displacement. This is a half-cycle separation, so $\Delta\phi = 2\pi(1/2) = \pi\ \text{rad}$ A: treats the two points as oscillating in phase. B: assigns a quarter-cycle separation instead of opposite-phase motion. D: assigns a three-quarter-cycle separation instead of a half-cycle. Remember: antinodes in neighbouring segments separated by a node oscillate in antiphase.