Step 1: State the concept.
Cellulose acetate phthalate (CAP) is a classic enteric coating polymer. Enteric coatings are meant to stay intact in the acidic stomach and dissolve only in the more alkaline intestine.
Step 2: Look at the chemistry.
CAP carries free carboxylic acid groups from its phthalate part. In the low pH of the stomach these stay un-ionised and the polymer is insoluble, protecting the drug.
Step 3: Reason about the intestinal environment.
When the dosage form reaches the small intestine, where pH rises above about $6$, the carboxyl groups ionise. The ionised polymer becomes soluble, the coat dissolves, and the drug is released.
Step 4: Eliminate pH independent solubility.
Option 1 is wrong because the whole point of CAP is that its solubility depends strongly on pH, not that it is constant.
Step 5: Eliminate erosion and zero order.
Option 3 (erosion) describes matrix wearing away, which is not the CAP mechanism. Option 4 (zero order) is a kinetic profile, not the reason a coat dissolves, so both are rejected.
Step 6: Conclude.
Drug release from CAP is governed by pH dependent solubility, which is option 2.
\[ \boxed{\text{Option 2: pH dependent solubility}} \]