Final answer:
The phase with the highest potassium permeability in the ventricular muscle action potential is the repolarization phase.
Step-by-step explanation:
In the ventricular muscle action potential, the phase with the highest potassium permeability is the repolarization phase.
During repolarization, K+ channels open and allow potassium ions to exit the cell, causing the membrane potential to become more negative.
This phase is important for restoring the resting membrane potential and preparing the cell for another action potential.