Step 1: Build the forced chains for each taste letter.
There are only three taste pairs possible: A-B, A-C, B-C. Look at what each one locks out from W, X, Y, Z. A-B locks out W. A-C locks out Y. B-C locks out both W and Y.
Step 2: Read off what happens whenever Y appears.
Y can only survive under the A-B pair, since A-C and B-C both remove Y. So using Y forces the taste pair to be A-B, meaning B always comes along. This confirms statement II is always true.
Step 3: Read off what happens whenever C is dropped.
If C is out, the only taste pair left from A, B, C is A-B, which locks out W. So dropping C always locks out W too, confirming statement III is always true.
Step 4: Check statement I with the same chain.
C appears in both A-C, which allows W, and B-C, which locks out W. Since one case allows W and the other blocks it, using C does not always force W in, so statement I fails.
Final Answer:
Only statements II and III are guaranteed every time.
\[ \boxed{\text{II and III only}} \]