If "undulate" means to move in a wave-like motion:
* Straighten: This implies making something flat or level.
* Straighten out: This implies making something smooth and without curves.
* Level: This implies making something flat and horizontal.
* Still: This implies a lack of movement or motion.
If "undulate" means to rise and fall in a rhythmic pattern:
* Constant: This implies a steady and unchanging state.
* Steady: This implies a smooth and even flow.
* Uniform: This implies a consistent and unchanging pattern.
If "undulate" means to have a wavy or undulating shape:
* Straight: This implies a lack of curves or bends.
* Smooth: This implies a lack of unevenness or roughness.
* Flat: This implies a lack of any rise or fall.
The best antonym will depend on the specific meaning of "undulate" in the context of your sentence.