C

Step 1: Understand stationary waves. Stationary waves are formed by the superposition of two waves of the same frequency and amplitude traveling in opposite directions.
Step 2: Identify the characteristics of antinodes. In stationary waves, the antinodes are the points where the displacement of the medium is at its maximum.
Step 3: Analyze pressure changes. At these points (antinodes), because the displacement is maximum, the pressure change is also maximum, as the density fluctuations in the medium are directly related to the displacement fluctuations.
Step 4: Conclusion. Thus, at the antinodes, there is maximum displacement and maximum pressure change.
Therefore, the correct answer is C.