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 Protectants [Clinical Pharmacy Guide - Module 5 - Supportive Care] for more information.