Leukocytosis with anaemia, thrombocytopenia and >20% blasts on the smear strongly suggests acute leukaemia. A bone marrow biopsy is required for diagnosis: it confirms blast proliferation (>20%) and defines the lineage (ALL vs AML).

• Iron is irrelevant here.
• Transfusion and antibiotics are supportive measures and do not establish the diagnosis. The sources do say fever in a known acute leukaemia should be treated as infection, so this answer depends on reading the stem as a diagnostic "next step".