The correct answer is 2. A lateral step-down requires the patient to perform hip abduction and adduction, which are frontal plane motions.
An anterior lunge requires the patient to perform hip flexion and extension (when returning from the lunge position), which are sagittal plane motions.
A posterior step-up requires the patient to perform hip flexion and extension, which are sagittal plane motions.
A posterior step down, regardless of the step height, requires the patient to perform hip flexion and extension, which are sagittal plane motions.