The correct answer is Interferon-gamma (IFN-γ) because it is the primary cytokine responsible for classical (M1) activation of macrophages. IFN-γ, produced mainly by Th1 cells and NK cells, enhances macrophage microbicidal functions by stimulating increased production of reactive oxygen species and nitric oxide. This activation leads to enhanced antigen presentation and pro-inflammatory cytokine secretion, crucial for effective immunity against intracellular pathogens. Other cytokines like IL-4 and IL-10 promote alternative (M2) macrophage activation, which is anti-inflammatory.

Reference: Janeway’s Immunobiology, 9th Edition, Chapter 7 - The Innate Immune System