Oncogenes with gain of function mutation lead to increased transcription, etc., whereas tumor suppressor genes block G1-->S phase. NF1 gene product has RAS GTPase activity which works by phosphorylating and activating protein (neurofibromin). So there is at least 1 tumor suppressor gene that works through phosphorylation.
This is essentially a formal logic question. Logically speaking, the question asks us to identify a mechanism that tumor suppressors have which proto-oncogenes do not. In other words, what is a mechanism shared by all known tumor suppressors but not shared by any known proto-oncogenes? For that reason, it can’t be phosphorylation; sure, phosphorylation is a mechanism of tumor suppressors but it’s also a mechanism of many known proto-oncogenes.