The correct answer is 3.
Rationale: Leucovorin reduces the toxicity of methotrexate by "rescuing" normal cells. In theory, it has been suggested that leucovorin has a preference for normal cells over certain tumour cells, due to a difference in their membrane transport mechanisms. This preference allows leucovorin to "rescue" normal cells from the toxic effects of methotrexate. See the Clinical Pharmacy Guide, Supportive Care Medications, Methotrexate Rescue section for more information.