Step 1: Understanding the Concept
A pistil accepts or rejects pollen after recognising it.
Step 2: Mechanism
Compatible pollen is accepted and incompatible pollen is rejected.
This recognition depends on special proteins in the pollen wall and the stigma surface.
Step 3: Match
The answer is special proteins, option (D).
Final Answer:
Special proteins decide compatibility and incompatibility.
\[ \boxed{\text{(D) }\text{special proteins}} \]