C

Step 1: Analyze Option A - Free fall is indeed one-dimensional motion as it only involves vertical motion under gravity.
Step 2: Analyze Option B - Projectile motion is a two-dimensional motion as it involves both horizontal and vertical components.
Step 3: Analyze Option C - Circular motion is primarily considered to be a two-dimensional motion since the motion occurs in a plane, although it can be described in three-dimensional space. However, it's incorrect to classify it as three-dimensional motion.
Step 4: Analyze Option D - Motion along a straight line is correctly classified as one-dimensional motion.
Therefore, the incorrect pair is Option C.