Reduced levels of natural killer cell ______________ have been found in people who are highly stressed, including students facing important examinations, bereaved persons, and those who are severely depressed.

Cytotoxicity

The correct answer is Option 3: Cytotoxicity

Reduced levels of natural killer cell cytotoxicity have been found in people who are highly stressed, including students facing important examinations, bereaved persons, and those who are severely depressed.