Final answer:
The most appropriate action is to assess the client for calf pain, warmth, and redness, as a platelet count of 9800/mm3 indicates severe thrombocytopenia.
Step-by-step explanation:
The most appropriate action by the nurse when a client has a platelet count of 9800/mm3 is to assess the client for calf pain, warmth, and redness (option A). A platelet count of 9800/mm3 indicates severe thrombocytopenia, which can lead to spontaneous bleeding, particularly in the lower extremities. Assessing for calf pain, warmth, and redness is important to monitor for signs of deep vein thrombosis (DVT), which is a potential complication of severe thrombocytopenia.