Natural Killer (NK) cells target virus-infected cells effectively when the infected cell __________?

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Displays MHC class I molecules on its surfaceFails to present MHC class I moleculesShows MHC class II molecules externallyLacks expression of MHC class II molecules

Explanation

NK cells recognize and kill infected cells that have reduced or absent MHC class I expression. Many viruses downregulate MHC class I to evade cytotoxic T cells, but this makes them susceptible to NK cell-mediated destruction. Therefore, NK cells are effective when the infected cell is unable to express MHC class I molecules.